Add basic contributing doco

Author: realityforge

CONTRIBUTING.md

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+# How to Contribute
+
+Pull requests are greatly appreciated and are what makes opensource great. Here's a quick guide:
+
+* Fork it
+* Create your feature branch (`git checkout -b my-new-feature`)
+* Commit your changes (`git commit -am 'Add some feature'`)
+* Push to the branch (`git push origin my-new-feature`)
+* Create new Pull Request
+
+Pester us if we don't get your Pull Requests merged in a timely fashion. :)
+
+## How to speed the merging of pull requests
+
+* Describe your changes in the CHANGELOG.
+* Give yourself some credit in the appropriate place (usually the CHANGELOG).
+* Make commits of logical units.
+* Ensure your commit messages help others understand what you are doing and why.
+* Check for unnecessary whitespace with `git diff --check` before committing.
+* Maintain the same code style.
+* Maintain the same level of test coverage or improve it.
+
+## Additional Resources
+
+* [General GitHub documentation](http://help.github.com/)
+* [GitHub pull request documentation](http://help.github.com/send-pull-requests/)