Add a Gitter chat badge to README.md

iNextrix/ASTPP now has a Chat Room on Gitter

[~smrdoshi] has just created a chat room. You can visit it here: https://gitter.im/ASTPP-Official/Lobby.

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

![Gitter(https://gitter.im/ASTPP-Official/Lobby?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=body_badge)

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.