Add contributing guidelines

CONTRIBUTING.md

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+## Contributing
+
+Contributions are welcome, and they are greatly appreciated! Every
+little bit helps, and credit will always be given. There a just a few small
+guidelines you need to follow.
+
+
+## Submitting a patch
+
+  1. It's generally best to start by opening a new issue describing the bug or
+     feature you're intending to fix.  Even if you think it's relatively minor,
+     it's helpful to know what people are working on.  Mention in the initial
+     issue that you are planning to work on that bug or feature so that it can
+     be assigned to you.
+
+  2. Follow the normal process of [forking][] the project, and setup a new
+     branch to work in.  It's important that each group of changes be done in
+     separate branches in order to ensure that a pull request only includes the
+     commits related to that bug or feature.
+
+  3. Go makes it very simple to ensure properly formatted code, so always run
+     `go fmt` on your code before committing it.
+
+  4. Any significant changes should almost always be accompanied by tests.  The
+     project already has good test coverage, so look at some of the existing
+     tests if you're unsure how to go about it.
+
+  5. Do your best to have [well-formed commit messages][] for each change.
+     This provides consistency throughout the project, and ensures that commit
+     messages are able to be formatted properly by various git tools.
+
+  6. Finally, push the commits to your fork and submit a [pull request][].
+
+[forking]: https://help.github.com/articles/fork-a-repo
+[well-formed commit messages]: http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html
+[squash]: http://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits
+[pull request]: https://help.github.com/articles/creating-a-pull-request
+