CONTRIBUTING.md: remove note about not accepting Pull Requests

Updates golang/go#24185

Change-Id: Ib544ebd3a7b3f2bd621e5054235a174358909042
Reviewed-on: https://go-review.googlesource.com/100693
Reviewed-by: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>