doc: add CONRIBUTORS.md

This file is known to GitHub and will be linked prominently when
potential contributors try to open pull requests.

Here's how it should appear:
	https://gist.github.com/adg/e0ffcc922a32f20af3db

Change-Id: I41c5aadf284a0a24c485f96b7fe2caa338ad1e4f
Reviewed-on: https://go-review.googlesource.com/1178
Reviewed-by: Rob Pike <r@golang.org>