Update README to reflect Play Store location.

README.md

@@ -39,6 +39,13 @@ Interested in helping to translate TextSecure? Contribute here:
 
 https://www.transifex.com/projects/p/textsecure-official/
 
+Downloads
+------------
+
+TextSecure can be downloaded from the Play Store here:
+
+https://play.google.com/store/apps/details?id=org.thoughtcrime.securesms
+
 Cryptography Notice
 ------------