Add building instruction to README (#3782)

Author: ralf0131

README.md

@@ -162,6 +162,14 @@ The consumer will print out `Hello world` on the screen.
 * [Dubbo developer guide](http://dubbo.apache.org/en-us/docs/dev/build.html) - How to involve in Dubbo development.
 * [Dubbo admin manual](http://dubbo.apache.org/en-us/docs/admin/install/provider-demo.html) - How to admin and manage Dubbo services.
 
+## Building
+
+If you want to try out the cutting-edge features, you can built with the following commands. (Java 1.8 is required to build the master branch)
+
+```
+  mvn clean install
+```
+
 ## Contact
 
 * Mailing list: