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

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

Change-Id: Ia54daec1cee91bd37510d07fb509174ac1344f4c
Reviewed-on: https://go-review.googlesource.com/c/website/+/275193
Trust: Julie Qiu <julie@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/README.md b/README.md
index 925faba..089acd7 100644
--- a/README.md
+++ b/README.md
@@ -1,5 +1,7 @@
 # Go website
 
+[![Go Reference](https://pkg.go.dev/badge/golang.org/x/website.svg)](https://pkg.go.dev/golang.org/x/website)
+
 This repository holds the Go website server code and content.
 
 ## Checkout and Run