message: Update secp256k1 README
  Summary:
  Now that secp256k1 is being bundled as it's own repo on Github,
  it would be nice to have a more accurate and complete README.
  Test Plan: Read it, make sure it's complete and accurate.
  Reviewers: deadalnix, markblundeberg, jasonbcox, #bitcoin_abc
  Subscribers: Fabien
diff: libsecp256k1
  ============
  -[![Build Status](https://travis-ci.org/bitcoin-core/secp256k1.svg?branch=master)](https://travis-ci.org/bitcoin-core/secp256k1)
  +[![Build Status](https://travis-ci.org/bitcoin-abc/secp256k1.svg?branch=master)](https://travis-ci.org/bitcoin-abc/secp256k1)
  -Optimized C library for EC operations on curve secp256k1.
  +Optimized C library for cryptographic operations on curve secp256k1.
  -This library is a work in progress and is being used to research best practices. Use at your own risk.
  +This library is used for consensus critical cryptographic operations on the Bitcoin Cash network. It is maintained within the Bitcoin ABC repository, and is mirrored as a separate repository for ease of reuse in other Bitcoin Cash projects. Developers who want to contribute may do so at [reviews.bitcoinabc.org](https://reviews.bitcoinabc.org/). Use at your own risk.
  Features:
  * secp256k1 ECDSA signing/verification and key generation.
  +* secp256k1 Schnorr signing/verification ([Bitcoin Cash Schnorr variant](https://www.bitcoincash.org/spec/2019-05-15-schnorr.html)).
  * Adding/multiplying private/public keys.
  * Serialization/parsing of private keys, public keys, signatures.
  * Constant time, constant memory access signing and pubkey generation.
  