Add IRC button to README

Idea from @bumi

README.md

@@ -1,6 +1,8 @@
 Build status: [![Build Status](https://travis-ci.org/bitcoinj/bitcoinj.png?branch=master)](https://travis-ci.org/bitcoinj/bitcoinj)  
 Coverage status: [![Coverage Status](https://coveralls.io/repos/bitcoinj/bitcoinj/badge.png?branch=master)](https://coveralls.io/r/bitcoinj/bitcoinj?branch=master)
 
+Chat with us: [![Visit our IRC channel](https://kiwiirc.com/buttons/irc.freenode.net/bitcoinj.png)](https://kiwiirc.com/client/irc.freenode.net/bitcoinj)
+
 ### Welcome to bitcoinj
 
 The bitcoinj library is a Java implementation of the Bitcoin protocol, which allows it to maintain a wallet and send/receive transactions without needing a local copy of Bitcoin Core. It comes with full documentation and some example apps showing how to use it.