Add a Gitter chat badge to README.md

[gitter-badger]

liygheart/jfinalbbs now has a Chat Room on Gitter

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

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.

[TakWolf]

这个是机器人吗

[atjiu]

是的，我刚才在gitter.im里开通了一个room，然后gitter.im就给项目发了一个pr，就加了一个图片