Add README Google Play badge link

Author: yawnoc

README.md

@@ -4,8 +4,17 @@ A minimalist Chinese keyboard implementing the stroke input method (筆畫輸入
 
 ![Screenshot of the keyboard during stroke input.](stroke-input-screenshot.png)
 
-- Google Play: currently "in review".
-- F-Droid: coming soon
+[
+  <img
+    alt="Get it on Google Play."
+    src="https://play.google.com/intl/en_us/badges/static/images/badges/en_badge_web_generic.png"
+    width="240"
+    height="auto">
+][google-play]
+
+(F-Droid: coming soon)
+
+[google-play]: https://play.google.com/store/apps/details?id=io.github.yawnoc.strokeinput
 
 
 ## License