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

A badge to pkg.go.dev/golang.org/x/pkgsite is added to the README.

Change-Id: Ibb6d4cd63f34a4afcc842e37cdaa20058cb7b9fa
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/275300
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Trust: Julie Qiu <julie@golang.org>
diff --git a/README.md b/README.md
index e077ec0..ca93225 100644
--- a/README.md
+++ b/README.md
@@ -1,5 +1,7 @@
 # Pkg.go.dev
 
+[![Go Reference](https://pkg.go.dev/badge/golang.org/x/pkgsite.svg)](https://pkg.go.dev/golang.org/x/pkgsite)
+
 ## A site for discovering Go packages
 
 Pkg.go.dev is a website for discovering and evaluating Go packages and modules.