Add links to Google Play store. (nodejs#107)

Adding links for beta channel and for current released app.

README.rst

@@ -2,6 +2,15 @@
 Zulip for Android
 =================
 
+This is the Android client for `Zulip <http://www.zulip.org>`_.  It is
+available `in the Google Play store
+<https://play.google.com/store/apps/details?id=com.zulip.android>`_.
+If you have a taste for adventure, you can also `join our beta channel
+<https://play.google.com/apps/testing/com.zulip.android>`_ after which
+the Play store should auto-update you. (Caution: beta releases will
+have more bugs than final releases, including (potentially) security
+or data loss bugs.)
+
 This is a Gradle project, and can be built via the provided ``gradlew``
 or by using Android Studio.