CONTRIBUTING.md: remove note about not accepting Pull Requests

Updates golang/go#24185

Change-Id: I10753a1e9829a7b2f74c3e9bade62f8a1d4ef9e1
Reviewed-on: https://go-review.googlesource.com/100682
Reviewed-by: Ian Lance Taylor <iant@golang.org>
2 files changed