Add a Gitter chat badge to README.md

[gitter-badger]

drepl/drepl now has a Chat Room on Gitter

@wilzbach has just created a chat room. You can visit it here: https://gitter.im/drepl/drepl.

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.

[wilzbach]

argh - the bot isn't that smart. will open a PR manually :/