README.md: add badge to pkg.go.dev

Change-Id: I2324b1db9d9fcf35c52c6c109de277bc1c053644
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/275891
Trust: Julie Qiu <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Run-TryBot: Julie Qiu <[email protected]>

Author: julieqiu

README.md

@@ -1,5 +1,7 @@
 # Go support for Mobile devices
 
+[![Go Reference](https://pkg.go.dev/badge/golang.org/x/mobile.svg)](https://pkg.go.dev/golang.org/x/mobile)
+
 The Go mobile repository holds packages and build tools for using Go on mobile platforms.
 
 Package documentation as a starting point: