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

Change-Id: Ib182f3257938ce7233ed68436bbfb28ddd1053f1
Reviewed-on: https://go-review.googlesource.com/c/build/+/275234
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

golang/build,golang/build,golang/build,golang/build