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>

README.md

@@ -1,5 +1,7 @@
 # Go Time
 
+[![Go Reference](https://pkg.go.dev/badge/golang.org/x/time.svg)](https://pkg.go.dev/golang.org/x/time)
+
 This repository provides supplementary Go time packages.
 
 ## Download/Install