Add badges in README.md (#119)

README.md

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 # A collection of Elliptic Curves for ZkCrypto traits
 
+[![crates.io version](https://img.shields.io/crates/v/halo2curves.svg)](https://crates.io/crates/halo2curves)
+[![docs.rs availability](https://img.shields.io/docsrs/halo2curves?label=docs.rs)](https://docs.rs/halo2curves)
+[![Build status](https://img.shields.io/github/actions/workflow/status/privacy-scaling-explorations/halo2curves/ci.yml?branch=main)](https://github.com/privacy-scaling-explorations/halo2curves/actions)
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 This library provides efficient and flexible implementations of various halo2-friendly elliptic curves, originally implementing the BN256 curve with traits from the `zkcrypto` ecosystem,
 
 * [`zkcrypto/ff`](https://github.com/zkcrypto/ff)