Add a Gitter chat badge to README.md

[gitter-badger]

DotNetAnalyzers/PublicApiAnalyzer now has a Chat Room on Gitter

@sharwell has just created a chat room. You can visit it here: https://gitter.im/DotNetAnalyzers/PublicApiAnalyzer.

This pull-request adds this badge to your README.md:

If my aim is a little off, please let me know.

Happy chatting.

PS: Click here if you would prefer not to receive automatic pull-requests from Gitter in future.

[sharwell]

This badge was already added.