When people add new API and get an all.bash failure,
they often don't know about the API checker at all.
Point to the README in the failure message, to try to
help them find what they need to know.
Change-Id: I6b148ec414d212033b371357a5e8c6ab79bb50a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/449015
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org> Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>