Added Contributing.md.

CONTRIBUTING.md

+# How to contribute
+
+
+
+## Getting Started
+
+* Make sure you have a [GitHub account](https://github.com/signup/free)
+* Fork the repository on GitHub
+
+## Making Changes
+
+* Create a topic branch from where you want to base your work.
+  * This is usually the master branch.
+  * Only target other branches if you are certain your fix must be on that
+    branch.
+  * To quickly create a topic branch based on master; `git branch
+    my_topic_branch master` then checkout the new branch with `git
+    checkout my_topic_branch`.  Please avoid working directly on the
+    `master` branch.
+* Make commits of logical units.
+* Make sure your code conforms to the [styleguide][styleguide].
+* Check for unnecessary whitespace with `git diff --check` before committing.
+* Make sure your commit messages are in the proper format.
+
+````
+    One sentence summary.
+
+    Detailed description of the commit ...
+    ....
+````
+
+* TODO: Test your changes
+
+## Submitting Changes
+
+* Push your changes to a topic branch in your fork of the repository.
+* Submit a pull request to the main repository.
+
+# Additional Resources
+
+* [General GitHub documentation](http://help.github.com/)
+* [GitHub pull request documentation](http://help.github.com/send-pull-requests/)
+* [OGS Jenkins-CI server](https://svn.ufz.de/hudson/job/OGS-6/)
+* [OGS Styleguide][styleguide]
+
+[styleguide]: http://ufz.github.com/styleguide/cppguide.xml
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