readme: Add IRC channel

Author: real-or-random

README.md

@@ -2,6 +2,7 @@ libsecp256k1
 ============
 
 [![Build Status](https://api.cirrus-ci.com/github/bitcoin-core/secp256k1.svg?branch=master)](https://cirrus-ci.com/github/bitcoin-core/secp256k1)
+[![irc.libera.chat #secp256k1](https://img.shields.io/badge/irc.libera.chat-%23secp256k1-success)](https://web.libera.chat/#secp256k1)
 
 Optimized C library for ECDSA signatures and secret/public key operations on curve secp256k1.