add google play badge

README.md

@@ -1,6 +1,8 @@
 # renamer
 
-A batch file renamer written in flutter (dart).
+A bulk file renamer written in flutter (dart).
+
+<a href="https://play.google.com/store/apps/details?id=net.sunjiao.renamer"><img alt="Get it on Google Play" src="https://play.google.com/intl/en_us/badges/images/generic/en-play-badge.png" height=60px /></a>
 
 todo:
 - ~~Duplicate name check~~.(Done.)