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/1178Reviewed-by: Rob Pike <r@golang.org>

CONTRIBUTING.md

+The Go project is an open source project.
+For information, see: https://golang.org/
+
+Its canonical Git repository is located at https://go.googlecode.com/go.
+(There is a mirror of the repository at https://github.com/golang/go.)
+
+Report issues here: https://github.com/golang/go/issues
+
+Go is the work of hundreds of contributors. We appreciate your help!
+
+To contribute, please read the contribution guidelines:
+	http://golang.org/doc/contribute.html
+
+## We DO NOT accept pull requests.
+