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

Change-Id: Id2de42cc898214a03252177faf7224010d0e4c52
Reviewed-on: https://go-review.googlesource.com/c/blog/+/275302
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Trust: Julie Qiu <julie@golang.org>
diff --git a/README.md b/README.md
index fcacf6d..fb4ee53 100644
--- a/README.md
+++ b/README.md
@@ -1,5 +1,7 @@
 # Go Blog
 
+[![Go Reference](https://pkg.go.dev/badge/golang.org/x/blog.svg)](https://pkg.go.dev/golang.org/x/blog)
+
 This repository holds the Go Blog server code and content.
 
 ## Download/Install