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

Change-Id: I6155f92cf256296f3753d1068cd96c753d3f3d9b
Reviewed-on: https://go-review.googlesource.com/c/playground/+/275197
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Trust: Julie Qiu <julie@golang.org>

README.md

@@ -1,5 +1,7 @@
 # playground
 
+[![Go Reference](https://pkg.go.dev/badge/golang.org/x/playground.svg)](https://pkg.go.dev/golang.org/x/playground)
+
 This subrepository holds the source for the Go playground:
 https://play.golang.org/