Add first version of CONTRIBUTE.markdown [ci skip]

CONTRIBUTE.markdown

@@ -0,0 +1,28 @@
+# Guidelines for contributing
+
+## 1. Fork & Clone
+
+Since you probably don't have rights to the main repo, you should Fork it (big
+button up top). After that, clone your fork locally and optionally add an
+upstream:
+
+    git remote add upstream git@github.com:DatabaseCleaner/database_cleaner.git
+
+## 2. Make sure the tests run fine
+
+- `bundle install`
+- Copy `db/sample.config.yml` to `db/config.yml` and edit it
+- Make sure to create the databases specified in `db/config.yml`
+- Run the tests with `bundle exec rspec`
+
+Note that if you don't have all the supported databases installed and running,
+some tests will fail.
+
+## 3. Prepare your contribution
+
+This is all up to you but a few points should be kept in mind:
+
+- Please write tests for your contribution
+- Make sure that previous tests still pass
+- Push it to a branch of your fork
+- Submit a pull request