Add a Gitter chat badge to README.md

[gitter-badger]

nebulasio/go-nebulas now has a Chat Room on Gitter

@YipLY has just created a chat room. You can visit it here: https://gitter.im/nebulasio/go-nebulas.

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.

[fbzhong]

Cool pull request.

Let's accept this PR after we migrate from slack to gitter.

[Chayada2530]

Kk