Add a Gitter chat badge to README.rst

### devil-iiit/sympy now has a Chat Room on Gitter

@devil-iiit has just created a chat room. You can visit it here: https://gitter.im/devil-iiit/sympy.

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

[![Gitter](https://badges.gitter.im/Join Chat.svg)](https://gitter.im/devil-iiit/sympy?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)

Happy chatting.