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

A badge is added to the x/website README to link to
pkg.go.dev/golang.org/x/website.

Change-Id: Ia54daec1cee91bd37510d07fb509174ac1344f4c
Reviewed-on: https://go-review.googlesource.com/c/website/+/275193
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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