CONTRIBUTING.md: remove note about not accepting Pull Requests
CONTRIBUTING.md: remove note about not accepting Pull Requests

Updates golang/go#24185

Change-Id: I74b024457ece54c6b02c3b017cb003142b75b4f8
Reviewed-on: https://go-review.googlesource.com/100690
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/review