Add a contribution guide as recognized by gitlab

Mostly C&P from the User wiki, with adjustments
to the new workflow.

CONTRIBUTING.md

+Contributing to the Dune Core Modules
+=====================================
+
+You've squashed an annoying bug or implemented a nifty new feature in DUNE?
+And you're willing to share your improvements with the community? This page
+explains how to get those changes to us and what to take care of.
+
+Take a look at the DUNE coding style
+------------------------------------
+
+Your work will enjoy much smoother sailing if you take a look at the Coding
+Style and try to stick to it with your changes. We understand that everyone
+has their personal preferences and that there is no such thing as the *right*
+coding style (in the end, it's a matter of taste), but DUNE is a pretty large
+project, and a consistent way of doing things really helps a lot when trying
+to find your way around a body of code as big as DUNE.
+
+Make sure to install the Whitespace Hook before starting to work, because
+our repositories enforce certain rules about whitespace and will not accept
+commits that violate those rules. And a developer will be much more motivated
+to merge your patch if doing so does not involve fixing a bunch of tab-based
+indentations that you inadvertently added as part of your changes
+
+Use Git to your advantage
+-------------------------
+
+We know, Git can be a bit daunting at first, but trust us, it's really worth
+investing half an hour to learn the basics! Even though you don't have any
+commit rights to the DUNE repositories, Git still allows you to create local
+commits on your machine, avoiding the usual ugly business of creating backup
+copies, copying around code in files, commenting and uncommenting variants etc.
+And when you're done and send the changes to us, we can simply import those
+commits into our repositories. That saves a lot of time and when your changes
+can be applied in five minutes using two or three commands, chances are a
+developer will much more easily find the time to do so. Git is really popular,
+so there are tons of tutorials all over the web. Here are some pointers:
+
+* [This](http://try.github.io/) is a very quick, hands-down introduction
+  to Git that allows you to try out Git directly in your browser.
+  Requires a GitHub account to continue at some point, though.
+* [This](http://git-scm.com/book) is a very well-written and detailed resource
+  for all things Git. Chapter 2 is a great introduction to Git that also explains
+  a little bit how Git works, which really helps to reduce the number of
+  *WTF just happened?* moments. ;-)
+* [This](http://eagain.net/articles/git-for-computer-scientists/) is a short and
+  sweet explanation of what Git does at a fundamental level - just the thing for
+  scientists! ;-)
+* [This](http://git-scm.com/doc/ext) is a collection of both introductory and
+  more in-depth Git resources.
+
+Whatever you do, make sure to set your Git identity so that the commits tell us who authored them!
+
+Getting the changes to us
+-------------------------
+
+You should get your changes to us in the following way:
+* Get an account for [our GitLab instance](htpp://gitlab.dune-project.org).
+* Fork the core module that you want to contribute to, just
+  as you would do on GitHub.
+* Push your changes to your fork on some branch.
+* Open a merge request using the branch you pushed your changes
+  to as the source branch and the master of the core module repository
+  as the target branch. GitLab will usually ask you about opening
+  a merge request if you browse it right after pushing to some branch.
+* Follow the discussion on the merge request to see what improvements
+  should be done to the branch before merging.
+
+If you have any questions or complaints about this workflow of
+contributing to Dune, please rant on the
+[dune-devel mailing list](mailto:dune-devel@dune-project.org).