Add play store app banner in the read me.
# edX Android
+[<img align="right" alt="Get it on Google Play" height="128" src="https://play.google.com/intl/en_us/badges/images/generic/en_badge_web_generic.png">](https://play.google.com/store/apps/details?id=org.edx.mobile)
+
This is the source code for the edX mobile Android app. It is changing rapidly
and its structure should not be relied upon. See http://code.edx.org for other
parts of the edX code base.
