Tell folks how they can contribute to the project.

CONTRIBUTING.md

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+# Contribute
+
+To contribute to ``nagios-herald``, you need to follow a few easy steps:
+
+1. Fork the repo.
+2. Clone your fork.
+3. Hack on your favorite bits like the formatters, helpers, etc.
+4. If you are adding new functionality, document it in its own file under ``docs/``.
+5. If necessary, rebase your commits into logical chunks, without errors.
+6. Verify your code by running the test suite, and adding additional tests if you can.
+7. Push the branch up to GitHub.
+8. Send a pull request.
+
+We'll do our best to get your changes in!
+
+## How to Run Tests
+
+The tests are written using Ruby's minitest.
+
+Name your tests with the ``test_`` prefix and place them in the ``test/unit/``
+directory.
+
+When you're ready to run your tests, simply run ``rake``:
+
+```
+% rake test
+```
+