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

Change-Id: I4e7431d8f45314c8504dece3180d7d725a998f21
Reviewed-on: https://go-review.googlesource.com/c/lint/+/276014
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/lint