README.md: add badge to pkg.go.dev
Change-Id: Ia7262e29ce9e1635a693d1f97b54e5d3671e55c2
Reviewed-on: https://go-review.googlesource.com/c/time/+/276015
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>