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

Change-Id: I48dc1611bd6e85697a64afb66aa6ae79a60adfce
Reviewed-on: https://go-review.googlesource.com/c/arch/+/275933
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

golang/arch,golang/arch