FIELD OF THE INVENTION The present invention, in general, relates to estimation of the strength of documents in a document portfolio. More specifically, it relates to the relative strength evaluation of a document with respect to a given reference document. BACKGROUND The intellectual property of a firm is an important indicator of the firm's performance in terms of its innovation practices and its prospects of future growth. Intellectual Property such as patents are not only used for providing a legal protection to the inventor(s) or assignee(s) by keeping competitors out of specific markets but also for generating revenue by out-licensing, cross-licensing, and franchising the IP rights. The increase in IP licensing activities has resulted in a phenomenal increase in the number of IP related litigation suits. Any patent related litigation requires in-depth evaluation of the patent and the related existing arts to determine the strength of the patent or the existing art. For example, consider patent invalidity analysis, where a party to the case tries to invalidate an existing patent of the other party to the case, a patent analyst needs to evaluate the strength of various prior art with respect to the patent involved in the lawsuit. Strength of prior art with respect to the patent signifies the extent of mapping or overlap of underlying concepts between the two. As another example, consider infringement analysis, where a party tries to prove the infrigement of its patent(s) by product(s) of the other party, an analyst needs to evaluate the strength of the product(s) with respect to the patent(s). As yet another example, consider patentability assessment, where an analyst tries to identify the novelty of a concept, the analyst needs to evaluate the strength of various prior art with respect to the concept to find out the extent of mapping of the prior art with the concept. Several methodologies exist in the art to evaluate the relevance of documents (patents or products) with respect to any reference document (patent or product). However, the existing methodologies suffer from drawbacks owing to the subjectiveness of interpretation based on personal judgement of the analysts. Multiple analysts analyzing the same document (patent or product) may evaluate its strength differently due to the difference in the understanding of the document. Further, assigning a higli, medium or low rating to tlie document maizes it difficult to compare the documents which are given an equal rating by an analyst, for minute details. Also, in certain cases, the analyst may be unable to classify the document as high or low relevance owing to the complexity of the document and classifies the document as medium. This results in an increase in the pool categorized as medium and subsequently may cause erroneous analysis. Further, the existing methodologies evaluate the strength of a patent by using criteria such as the underlying technology, the R&D costs, the market value of the patent, the expected income from the patent etc. Apart from these criteria, criteria such as the number of fonA/ard references, number of backward references, geographic coverage, the IPC classification, the number of dependent claims and independent claims, the average length of a claim, the remaining age of the patent etc. have also been used to evaluate the strength of the patent. However, the above mentioned methodologies do not evaluate the strength of the patent quantitatively based on mapping of the key concepts between patents. In light of the above limitations of existing art, there is a need to quantify the strength of the documents (patents or products) in a more objective and deterministic manner. Further, there is a need for a method to deduce the strength of the document that is independent of the analyst's subjective judgements. SUMMARY OF THE INVENTION The present invention provides a computer implemented method and system to estimate the relative strength of a document with respect to a reference document or a portion of the reference document. In an embodiment of the present invention, the estimation of relative strength of a document with respect to a reference document or a portion of the reference document is done quantitatively. Numerical value of strength is calculated for the document with respect to the reference document or the portion of the reference document. Examples of the document and the reference document include, but are not limited to, patents, publications and product manuals. An example of the portion of the reference document includes, but is not limited to, the claims of the patent. The method includes identification of a first set of technical features associated with the reference document. The method further includes identifying a second set of technical features that are associated with the portion of the reference document. A Relative Weight Index (RWI) is computed for each of the identified first set of technical features based on their relative importance. Thereafter, the Relative Weight Index (RWI) of each of the first set of technical features and a Feature Score (FS) of the second set of technical features are combined by using a Strength equation to compute the strength of the document. Feature Score (FS) represents the association of the technical features of the reference document, with the document whose strength is analyzed. In an alternative embodiment of the invention, a method of creating a target analysis document Is disclosed. The target analysis document indicates the relative strength of a document with respect to a portion of a reference document. The method includes storing a first set of technical features associated with the reference document in the target analysis document. The method also includes identifying a second set of technical features associated with the portion of the reference document. A Relative Weight Index (RWI) Is computed for each of the identified first set of technical features based on their relative Importance. Thereafter, the Relative Weight Index (RWI) is combined with the Feature Score (FS) of the second set of technical features using a strength equation to compute a Significance Index (SI) of the document. The Significance Index (SI) indicates the strength of the document with respect the portion of the reference document. The target analysis document Is modified to Include the Significance Index (SI). The target analysis document may be used by a user for Intellectual property analysis. In yet another embodiment of the invention, a system for determining relative strength of a document in a set of documents with respect to a portion of a reference document Is disclosed. The system includes an Input device for receiving relative importance of each of the first set of technical features from a user. The system also Includes a memory for storing a first set of technical features of the reference document. The memory also stores a second set of technical features associated with the portion of the reference document. Further, a processor Is Included In the system for computing Relative Weight Index (RWI) for each of the first set of technical features from the relative importance by using a mathematical model. The processor also computes a Significance Index (SI) of the document based on the Relative Weight Index (RWI) of each of the first set of technical features. The system includes an output device for displaying tlie Significance Index of the document. The Significance Index (SI) of the document represents the relative strength of the document in the set of documents v\/ith respect to the portion of the reference document. BRIEF DESCRIPTION OF THE DRAWINGS The preferred embodiments of the invention will hereinafter be described in conjunction with the appended drawings provided to illustrate and not to limit the invention, wherein like designations denote lil