Update README.md

README.md

@@ -1,5 +1,7 @@
 # RT-Thread #
 
+**The master branch is frozen util v3.0.0 has been updated**
+
 [![Build Status](https://travis-ci.org/RT-Thread/rt-thread.svg)](https://travis-ci.org/RT-Thread/rt-thread)
 [![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/RT-Thread/rt-thread?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)