Add a CONTRIBUTING file

This uses the content of the 'Contribute' wiki page.

CONTRIBUTING.md

@@ -0,0 +1,45 @@
+Contribute
+==========
+
+So you've got an awesome idea to throw into Jekyll. Great! Please keep the following in mind:
+
+* **Contributions will not be accepted without tests.**
+* If you're creating a small fix or patch to an existing feature, just a simple test will do. Please stay in the confines of the current test suite and use [Shoulda](http://github.com/thoughtbot/shoulda/tree/master) and [RR](http://github.com/btakita/rr/tree/master).
+* If it's a brand new feature, make sure to create a new [Cucumber](https://github.com/cucumber/cucumber/) feature and reuse steps where appropriate. Also, whipping up some documentation in your fork's wiki would be appreciated, and once merged it will be transferred over to the main wiki.
+
+Test Dependencies
+-----------------
+
+To run the test suite and build the gem you'll need to install Jekyll's dependencies. Jekyll uses Bundler, so a quick run of the bundle command and you're all set!
+
+    $ bundle
+
+Before you start, run the tests and make sure that they pass (to confirm your environment is configured properly):
+
+    $ rake test
+    $ rake features
+
+Workflow
+--------
+
+Here's the most direct way to get your work merged into the project:
+* Fork the project
+* Clone down your fork ( `git clone git://github.com/<username>/jekyll.git` )
+* Create a topic branch to contain your change ( `git checkout -b my_awesome_feature` )
+* Hack away, add tests. Not necessarily in that order.
+* Make sure everything still passes by running `rake`
+* If necessary, rebase your commits into logical chunks, without errors
+* Push the branch up ( `git push origin my_awesome_feature` )
+* Create an issue with a description and link to your branch
+
+Gotchas
+-------
+
+* If you want to bump the gem version, please put that in a separate commit. This way, the maintainers can control when the gem gets released.
+* Try to keep your patch(es) based from the latest commit on mojombo/jekyll. The easier it is to apply your work, the less work the maintainers have to do, which is always a good thing.
+* Please don't tag your GitHub issue with [fix], [feature], etc. The maintainers actively read the issues and will label it once they come across it.
+
+Finally...
+----------
+
+Thanks! Hacking on Jekyll should be fun, and if for some reason it's a pain to do let us know so we can fix it.