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

Change-Id: I4e7431d8f45314c8504dece3180d7d725a998f21
Reviewed-on: https://go-review.googlesource.com/c/lint/+/276014
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>

README.md

diff --git a/README.md b/README.md
index 4968b13..ba4956f 100644
--- a/README.md
+++ b/README.md
@@ -1,5 +1,6 @@
 Golint is a linter for Go source code.
 
+[![Go Reference](https://pkg.go.dev/badge/golang.org/x/lint.svg)](https://pkg.go.dev/golang.org/x/lint)
 [![Build Status](https://travis-ci.org/golang/lint.svg?branch=master)](https://travis-ci.org/golang/lint)
 
 ## Installation