Add Go coding style guidelines

Signed-off-by: Stephen J Day <stephen.day@docker.com>

CONTRIBUTING.md

@@ -90,3 +90,39 @@ It's mandatory to:
 Complying to these simple rules will greatly accelerate the review process, and will ensure you have a pleasant experience in contributing code to the Registry.
 
 Have a look at a great, succesful contribution: the [Ceph driver PR](https://github.com/docker/distribution/pull/443)
+
+## Coding Style
+
+Unless explicitly stated, we follow all coding guidelines from the Go
+community. While some of these standards may seem arbitrary, they somehow seem
+to result in a solid, consistent codebase.
+
+The rules:
+
+1. All code should be formatted with `gofmt -s`.
+2. All code should pass the default levels of
+   [`golint`](https://github.com/golang/lint).
+3. All code should follow the guidelines covered at
+   https://github.com/golang/go/wiki/CodeReviewComments.
+4. Comment the code. Tell us the why, the history and the context.
+5. Document _all_ declarations and methods, even private ones. Declare
+   expectations, caveats and anything else that may be important. If a type
+   gets exported, having the comments already there will ensure it's ready.
+6. Variable name length should be proportional to it's context and no longer.
+   noALongVariableNameLikeThisIsNotMoreClearWhenASimpleCommentWouldDo. In
+   practice, short methods will have short variable names and globals will
+   have longer names.
+7. No underscores in package names. If you need a compound name, step back,
+   and re-examine why you need a compound name. If you still think you need a
+   compound name, lose the underscore.
+8. No utils or helpers packages. If a function is not general enough to
+   warrant it's own package, it has not been written generally enough to be a
+   part of a util package. Just leave it unexported and well-documented.
+9. No, we don't need another unit testing framework.
+10. Even though we call these "rules" above, they are actually just
+    guidelines. Since you've read all the rules, you now know that.
+
+If you are having trouble getting into the mood of idiomatic Go, we recommend
+reading through [`Effective Go`](http://golang.org/doc/effective_go.html). The
+[Go Blog](http://blog.golang.org/) is also a great resource. Drinking the
+kool-aid is a lot easier than going thirsty.