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

Change-Id: I4ea3e103c290e5d713a791178b35a41509e87275
Reviewed-on: https://go-review.googlesource.com/c/sync/+/275934
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

Author: julieqiu

README.md

@@ -1,5 +1,7 @@
 # Go Sync
 
+[![Go Reference](https://pkg.go.dev/badge/golang.org/x/sync.svg)](https://pkg.go.dev/golang.org/x/sync)
+
 This repository provides Go concurrency primitives in addition to the
 ones provided by the language and "sync" and "sync/atomic" packages.