Add contributing file.

CONTRIBUTING.md

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+# Contributing
+
+In the spirit of [free software][1], **everyone** is encouraged to help improve this project.
+
+Here are some ways *you* can contribute:
+
+* by using prerelease versions
+* by reporting [bugs][2]
+* by suggesting new features
+* by writing or editing documentation
+* by writing specifications
+* by writing code (*no patch is too small*: fix typos, add comments, clean up inconsistent whitespace)
+* by refactoring code
+* by resolving [issues][2]
+* by reviewing patches
+
+Starting point:
+
+* Fork the repo
+* Clone your repo
+* Run `bundle install`
+* Run `bundle exec rake test_app` to create the test application in `spec/dummy`
+* Make your changes
+* Ensure specs pass by running `bundle exec rspec spec`
+* Submit your pull request
+
+[1]: http://www.fsf.org/licensing/essays/free-sw.html
+[2]: https://github.com/mollie/spree-mollie-gateway/issues