Add CONTRIBUTING.md

See:

  * https://github.com/blog/1184-contributing-guidelines
  * http://lists.opensuse.org/yast-devel/2013-10/msg00052.html

CONTRIBUTING.md

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+YaST Contribution Guidelines
+============================
+
+YaST is an open source project and as such it welcomes all kinds of
+contributions. If you decide to contribute, please follow these guidelines to
+ensure the process is effective and pleasant both for you and YaST maintainers.
+
+There are two main forms of contribution: reporting bugs and performing code
+changes.
+
+Bug Reports
+-----------
+
+If you find a problem, please report it either using
+[Bugzilla](https://bugzilla.novell.com/enter_bug.cgi?format=guided&product=openSUSE+Factory&component=YaST2)
+or [GitHub issues](../../issues). (For Bugzilla, use the [simplified
+registration](https://secure-www.novell.com/selfreg/jsp/createSimpleAccount.jsp)
+if you don't have an account yet.)
+
+If you find a problem, please report it either using
+[Bugzilla](https://bugzilla.novell.com/) or GitHub issues. We can't guarantee
+that every bug will be fixed, but we'll try.
+
+When creating a bug report, please follow our [bug reporting
+guidelines](http://en.opensuse.org/openSUSE:Report_a_YaST_bug).
+
+Code Changes
+------------
+
+We welcome all kinds of code contributions, from simple bug fixes to significant
+refactorings and implementation of new features. However, before making any
+non-trivial contribution, get in touch with us first — this can prevent wasted
+effort on both sides. Also, have a look at our [development
+documentation](http://en.opensuse.org/openSUSE:YaST_development).
+
+To send us your code change, use GitHub pull requests. The workflow is as
+follows:
+
+  1. Fork the project.
+
+  2. Create a topic branch based on `master`.
+
+  3. Implement your change, including tests (if possible). Make sure you adhere
+     to the [Ruby style
+     guide](https://github.com/SUSE/style-guides/blob/master/Ruby.md).
+
+  4. Make sure your change didn't break anything by building the RPM package
+     (`rake osc:build`). The build process includes running the full testsuite.
+
+  5. Publish the branch and create a pull request.
+
+  6. YaST developers will review your change and possibly point out issues.
+     Adapt the code under their guidance until they are all resolved.
+
+  7. Finally, the pull request will get merged or rejected.
+
+See also [GitHub's guide on
+contributing](https://help.github.com/articles/fork-a-repo).
+
+If you want to do multiple unrelated changes, use separate branches and pull
+requests.
+
+Do not change the `VERSION` and `*.changes` files as this could lead to
+conflicts.
+
+### Commits
+
+Each commit in the pull request should do only one thing, which is clearly
+described by its commit message. Especially avoid mixing formatting changes and
+functional changes into one commit. When writing commit messages, adhere to
+[widely used
+conventions](http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html).
+
+If your commit is related to a bug in Buzgilla or an issue on GitHub, make sure
+you mention it in the commit message for cross-reference. Use format like
+bnc#775814 or gh#yast/yast-foo#42. See also [GitHub
+autolinking](https://help.github.com/articles/github-flavored-markdown#references)
+and [openSUSE abbreviation
+reference](http://en.opensuse.org/openSUSE:Packaging_Patches_guidelines#Current_set_of_abbreviations).
+
+Additional Information
+----------------------
+
+If you have any question, feel free to ask at the [development mailing
+list](http://lists.opensuse.org/yast-devel/) or at the
+[#yast](http://webchat.freenode.net/?channels=%23yast) IRC channel on freenode.
+We'll do our best to provide a timely and accurate answer.