Import badge image for Google Play.

This is the English PNG downloaded directly from the linked page.

[greg: added THIRDPARTY section]

Author: jackrzhang

docs/THIRDPARTY

@@ -39,6 +39,16 @@ License:
  .
  Apple is a trademark, and App Store is a service mark, of Apple Inc.
 
+Files: static/img/google-play-badge.png
+Copyright: 2018 Google LLC
+License:
+ Non-free.  Provided by Google to promote content available on
+ Google Play.  Downloaded from, and see guidelines at:
+ .
+ https://play.google.com/intl/en_us/badges/
+ .
+ Google Play and the Google Play logo are trademarks of Google LLC.
+
 License: MIT
  Permission is hereby granted, free of charge, to any person obtaining a
  copy of this software and associated documentation files (the "Software"),