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

Change-Id: I10b527ce21a609b674aa517cccaecd75c5aeef2e
Reviewed-on: https://go-review.googlesource.com/c/net/+/275889
Trust: Julie Qiu <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Run-TryBot: Julie Qiu <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: julieqiu

README.md

@@ -1,5 +1,7 @@
 # Go Networking
 
+[![Go Reference](https://pkg.go.dev/badge/golang.org/x/net.svg)](https://pkg.go.dev/golang.org/x/net)
+
 This repository holds supplementary Go networking libraries.
 
 ## Download/Install