Add a Gitter chat badge to README.md

[gitter-badger]

swadey/Lisp.jl now has a Chat Room on Gitter

@swadey has just created a chat room. You can visit it here: https://gitter.im/swadey/Lisp.jl.

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.

[Ismael-VC]

@swadey thanks so much! :D Now some twitter foo...