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

Change-Id: I3f506a31382e1291faca9c134e0a09607464ded4
Reviewed-on: https://go-review.googlesource.com/c/debug/+/276013
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>
diff --git a/README.md b/README.md
index 9b4f8d2..8afae2a 100644
--- a/README.md
+++ b/README.md
@@ -1,5 +1,7 @@
 # Go Debug
 
+[![Go Reference](https://pkg.go.dev/badge/golang.org/x/debug.svg)](https://pkg.go.dev/golang.org/x/debug)
+
 This repository holds utilities and libraries for debugging Go programs.
 
 **WARNING!**