add discuss badge to readme
<a href="http://libp2p.io/"><img src="https://img.shields.io/badge/project-libp2p-yellow.svg?style=flat-square" /></a>
<a href="http://webchat.freenode.net/?channels=%23libp2p"><img src="https://img.shields.io/badge/freenode-%23libp2p-yellow.svg?style=flat-square" /></a>
<a href="https://waffle.io/libp2p/libp2p"><img src="https://img.shields.io/badge/pm-waffle-yellow.svg?style=flat-square" /></a>
+ <a href="https://discuss.libp2p.io"><img src="https://img.shields.io/discourse/https/discuss.libp2p.io/posts.svg"/></a>
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