doc: add CONTRIBUTING.md
Reviewed-by: Brad Fitzpatrick <firstname.lastname@example.org>
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+# Contributing to Go
+Go is an open source project.
+It is the work of hundreds of contributors. We appreciate your help!
+## Filing issues
+When [filing an issue](https://golang.org/issue/new), make sure to answer these five questions:
+1. What version of Go are you using (`go version`)?
+2. What operating system and processor architecture are you using?
+3. What did you do?
+4. What did you expect to see?
+5. What did you see instead?
+General questions should go to the [golang-nuts mailing list](https://groups.google.com/group/golang-nuts) instead of the issue tracker.
+The gophers there will answer or ask you to file an issue if you've tripped over a bug.
+## Contributing code
+Please read the [Contribution Guidelines](https://golang.org/doc/contribute.html)
+before sending patches.
+**We do not accept GitHub pull requests**
+(we use [Gerrit](https://code.google.com/p/gerrit/) instead for code review).
+Unless otherwise noted, the Go source files are distributed under
+the BSD-style license found in the LICENSE file.