docs(readme): add an introductory description

Author: cedoor

README.md

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         Sparse Merkle tree
-        <sub>In progress</sub>
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     <p align="center">Sparse Merkle tree implementation in TypeScript.</p>
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     <img alt="NPM bundle size" src="https://img.shields.io/bundlephobia/min/@cedoor/smt?style=flat-square">
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+A sparse Merkle tree (SMT) is a data structure useful for storing a key/value map where every leaf node of the tree contains the cryptographic hash of a key/value pair and every non leaf node contains the concatenated hashes of its child nodes. SMTs provides a secure and efficient verification of large data sets and they are often used in peer-to-peer technologies. This implementation is an optimized version of the traditional sparse Merkle tree and it is based on the concepts expressed in the papers and resources below.
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+**Notice**: this library is still not stable and therefore it must be used with care.
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 ## References
 
 1. Rasmus Dahlberg, Tobias Pulls and Roel Peeters. *Efficient Sparse Merkle Trees: Caching Strategies and Secure (Non-)Membership Proofs*. Cryptology ePrint Archive: Report 2016/683, 2016. https://eprint.iacr.org/2016/683.