Added a build status image

With the build status information everybody immediately knows if the build is stable or has issues, more information about that build can then be found by clicking on the link.

fixes #2352

Author: javto

README.md

@@ -1,5 +1,7 @@
 # WordPress for Android #
 
+[![Build Status](https://travis-ci.org/wordpress-mobile/WordPress-Android.svg?branch=develop)](https://travis-ci.org/wordpress-mobile/WordPress-Android)
+
 If you're just looking to install WordPress for Android, you can find
 it on [Google Play][1]. If you're a developer wanting to contribute,
 read on.