Added CONTRIBUTING.md (fixes #12)

Parts of the file are copied from:
- https://github.com/puppetlabs/puppet/blob/master/CONTRIBUTING.md
- https://github.com/ungoldman/CONTRIBUTING.md/blob/master/CONTRIBUTING.md

CONTRIBUTING.md

@@ -0,0 +1,56 @@
+# Contributing Guidelines
+
+## Notes
+
+- **Before spending lots of time on something, ask for feedback on your idea first!**
+- Please search issues and pull requests before adding something new to avoid duplicating efforts and conversations.
+- Issues which are considered to be easy to solve are marked with the label `easy`: [Browse all easy issues](https://github.com/LibrePCB/LibrePCB/labels/easy).
+
+## Getting Started
+
+- Make sure you have a [GitHub account](https://github.com/signup/free).
+- Submit a ticket for your issue, assuming one does not already exist.
+- Fork the repository on GitHub.
+- Have a look at our [development resources](https://github.com/LibrePCB/LibrePCB/tree/master/dev), 
+  especially at the [Doxygen documentation](http://librepcb.org/LibrePCB-Doxygen/master/).
+- When using QtCreator, import and use our [code style guide file](https://github.com/LibrePCB/LibrePCB/blob/master/dev/CodingStyle_QtCreator.xml).
+
+## Making Changes
+
+- Create a topic branch from where you want to base your work.
+  - This is usually the master branch.
+  - To quickly create a topic branch based on master: 
+    `git checkout -b my_contribution master`
+  - Please avoid working directly on the `master` branch.
+- Write code which follows our [code style guides](http://librepcb.org/LibrePCB-Doxygen/master/df/d24/doc_code_style_guide.html)
+  and [.editorconfig settings](https://github.com/LibrePCB/LibrePCB/blob/master/.editorconfig).
+- Make commits of logical units.
+  - Make sure your commit messages are in the proper format.
+    Example: [0d896b69fdffbeb873957ed2b66eb9b0aa2ba750](https://github.com/LibrePCB/LibrePCB/commit/0d896b69fdffbeb873957ed2b66eb9b0aa2ba750)
+- Make sure you have added the necessary tests for your changes.
+- Run _all_ the tests to assure nothing else was accidentally broken.
+
+## Submitting Changes
+
+- Push your changes to a topic branch in your fork of the repository.
+- Submit a pull request to the repository in the LibrePCB organization.
+- We will then check the pull request and give you feedback quickly.
+
+## Other Contributions
+
+This project welcomes non-code contributions, too! The following types of contributions are welcome:
+
+- **Ideas**: Participate in an issue thread or start your own to have your voice heard.
+- **Bugreports**: Found a bug in LibrePCB? Just open an issue and describe how to reproduce that bug.
+- **Small Fixes**: Fix typos, clarify language, and generally improve the quality of the content.
+- **Documentation**: Create/improve documentations for end-users or for developers of LibrePCB.
+- **Translations**: Add/improve [translations for LibrePCB](https://github.com/LibrePCB/LibrePCB/tree/master/i18n).
+- **Website**: Improve our [website](http://librepcb.org) which is hosted in the repository [LibrePCB/LibrePCB.github.io](https://github.com/LibrePCB/LibrePCB.github.io).
+- **Sharing**: Speak about LibrePCB with your friends and colleagues, or write about it in the internet!
+- **Donations**: Donate to the LibrePCB developers with Bitcoin: [1FiXZxoXe3px1nNuNygRb1NwcYr6U8AvG8](bitcoin:1FiXZxoXe3px1nNuNygRb1NwcYr6U8AvG8)
+
+# Additional Resources
+
+- [General GitHub documentation](https://help.github.com/)
+- [GitHub pull request documentation](https://help.github.com/send-pull-requests/)
+