Add contribution guidelines.

CONTRIBUTING.md

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+We love pull requests. Here's a quick guide:
+
+1. Fork the repo.
+
+2. Run the tests. We only take pull requests with passing tests, and it's great
+to know that you have a clean slate: `bundle && rake`
+
+3. Add a test for your change. Only refactoring and documentation changes
+require no new tests. If you are adding functionality or fixing a bug, we need
+a test!
+
+4. Make the test pass.
+
+5. Push to your fork and submit a pull request.
+
+
+At this point you're waiting on us. We like to at least comment on, if not
+accept, pull requests within three business days (and, typically, one business
+day). We may suggest some changes or improvements or alternatives.
+
+Some things that will increase the chance that your pull request is accepted,
+taken straight from the Ruby on Rails guide:
+
+* Use Rails idioms and helpers
+* Include tests that fail without your code, and pass with it
+* Update the documentation, the surrounding one, examples elsewhere, guides,
+  whatever is affected by your contribution
+
+Syntax:
+
+* Two spaces, no tabs.
+* No trailing whitespace. Blank lines should not have any space.
+* Prefer &&/|| over and/or.
+* MyClass.my_method(my_arg) not my_method( my_arg ) or my_method my_arg.
+* a = b and not a=b.
+* Follow the conventions you see used in the source already.
+
+And in case we didn't emphasize it enough: we love tests!