README.md: add badge to pkg.go.dev
A badge to pkg.go.dev/golang.org/x/pkgsite is added to the README.
Trust: Julie Qiu
# Pkg.go.dev
+[![Go Reference](https://pkg.go.dev/badge/golang.org/x/pkgsite.svg)](https://pkg.go.dev/golang.org/x/pkgsite)
+
## A site for discovering Go packages
Pkg.go.dev is a website for discovering and evaluating Go packages and modules.
