Add a Gitter chat badge to README.md
@@ -1,4 +1,5 @@
# RT-Thread #
+[](https://gitter.im/RT-Thread/rt-thread?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
[](https://travis-ci.org/RT-Thread/rt-thread)
[](https://gitter.im/RT-Thread/rt-thread)