Add CONTRIBUTON file

CONTRIBUTING.md

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+# Contributing
+
+Thank you for helping radish to get a better piece of software.
+
+## Support
+
+If you have any questions regarding the usage of radish as a BDD tool please use [StackOverflow](https://stackoverflow.com).
+
+## Reporting Issues / Proposing Features
+
+Before you submit an Issue or proposing a Feature check the existing Issues in order to avoid duplicates. <br>
+Please make sure you provide enough information to work on your submitted Issue or proposed Feature:
+
+* Which version of radish are you using?
+* Which version of python are you using?
+* On which platform are you running radish?
+
+## Pull Requests
+
+We are very happy to receive Pull Requests considering:
+
+* Style Guide. Follow the rules of [PEP8](http://legacy.python.org/dev/peps/pep-0008/), but you may ignore *too-long-lines* and similar warnings. There is a *pylintrc* file for more information.
+* Tests. If our change affects python code inside the source code directory, please make sure your code is covered by an automated test case.
+
+### Testing
+
+To test the radish source code against all supported python versions you should use *tox*:
+
+```bash
+cd ~/work/radish
+pip install tox
+tox
+```
+
+However, if you want to test your code on certain circumstances you can create a *virtualenv*:
+
+```
+cd ~/work/radish
+virtualenv env
+source env/bin/activate
+pip install -r development.txt
+pip install .
+nosetests --rednose -v tests/unit --with-cover --cover-package=radish/
+```