Merge pull request #1792 from redisson/jackygurui-patch-1

added a section explaining building branch

CONTRIBUTING.md

@@ -3,6 +3,11 @@ Have at least a local copy of built redis, for more information see [tutorial](h
 
 Note that redis shouldn't be running - the build will start instances as needed.
 
+
+## Choosing the building branch
+Prior to the build, you may need to change branch depending on which version of Redisson you want to build. Currently master branch is for version 2.x whereas 3.0.0 branch is for 3.x branch. Version 2.x requies Java SDK 6 and above and version 3.x requires Java SDK 8 and above to build.
+
+
 ## Running the tests ##
 
 ``` bash