Add contributing infos

CONTRIBUTING.md

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+# How to contribute
+We would love to have your help. Before you start working however, please read
+and follow this short guide.
+
+# Translation
+Please go to our [translation website](http://translate.jitsi.org), which is a
+[Pootle](http://pootle.translatehouse.org/) instance that allows for easy
+online translation. Many languages are already there and wait for completion.
+
+If your preferred language is not yet created, please drop us an
+[e-mail](mailto:dev@jitsi.org). Be aware though that completing a new language
+will be quite some effort, so be sure you actually want to take up on that
+work and translate more than a few words.
+
+Please do not create any pull requests for the resources_XX.properties files,
+they will be overwritten with what comes from
+[Pootle](http://translate.jitsi.org). The only exception is if you find
+mistakes in the language file for
+[English](https://github.com/jitsi/jitsi/blob/master/resources/languages/resources.properties).
+
+For some more information, e.g. how to translate offline visit the
+[How To Translate Jitsi](https://jitsi.org/Documentation/HowToTranslateSIPCommunicator)
+on our website.
+
+# Code contributions
+Found a bug and know how to fix it? Great! Please read on.
+
+## Contributor License Agreement
+While Jitsi is released under the
+[LGPL](https://github.com/jitsi/jitsi/blob/master/LICENSE), the copyright
+holder and principal creator is [Blue Jimp](http://www.bluejimp.com/). To
+ensure that we can continue making Jitsi available under an open source license
+and still pay our bills, we need you to sign our
+[contributor license agreement](http://www.bluejimp.com/bca.pdf). If you
+cannot accept the terms laid out in the agreement, unfortunately, we cannot
+accept your contribution.
+
+## Coding Rules
+- Please read and follow the [code conventions](https://jitsi.org/Documentation/CodeConvention),
+  especially the limit on 80 characters per line.
+- Do not reformat existing code.
+
+## Creating Pull Requests
+- Perform **one** logical change per pull request.
+- Maintain a clean list of commits, squash them if necessary.
+- Rebase your topic branch on top of the master branch before creating the pull request.