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

Updates golang/go#24185

Change-Id: I1ce066be52feff5d2372633776aa631e0ce7e713
Reviewed-on: https://go-review.googlesource.com/100684
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/mobile,golang/mobile,golang/mobile,golang/mobile