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

Updates golang/go#24185

Change-Id: I228e34ceb525d5d41d1146ef17d6f958a143026e
Reviewed-on: https://go-review.googlesource.com/100681
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/exp,golang/exp,golang/exp