Adding Go coding style guidelines

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

CONTRIBUTING.md

@@ -234,6 +234,8 @@ close an issue. Including references automatically closes the issue on a merge.
 Please do not add yourself to the `AUTHORS` file, as it is regenerated regularly
 from the Git history.
 
+Please see the [Coding Style](#coding-style) for further guidelines.
+
 ### Merge approval
 
 Docker maintainers use LGTM (Looks Good To Me) in comments on the code review to
@@ -385,3 +387,49 @@ do need a fair way to deal with people who are making our community suck.
 	appeals, we know that mistakes happen, and we'll work with you to come up with a
 	fair solution if there has been a misunderstanding.
 
+## 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.
+
+It is possible that the code base does not currently comply with these
+guidelines. We are not looking for a massive PR that fixes this, since that
+goes against the spirit of the guidelines. All new contributions should make a
+best effort to clean up and make the code base better than they left it.
+Obviously, apply your best judgement. Remember, the goal here is to make the
+code base easier for humans to navigate and understand. Always keep that in
+mind when nudging others to comply.
+
+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 in [Effective
+   Go](http://golang.org/doc/effective_go.html) and [Go Code Review
+   Comments](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.
+   `noCommaALongVariableNameLikeThisIsNotMoreClearWhenASimpleCommentWouldDo`.
+   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. All tests should run with `go test` and outside tooling should not be
+   required. No, we don't need another unit testing framework. Assertion
+   packages are acceptable if they provide _real_ incremental value.
+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.