Create CODE_OF_CONDUCT.md

By adding (a link to) our code of conduct to the github repo, Github will be able to show it to people. Yay!

CODE_OF_CONDUCT.md

+In the Nextcloud community, participants from all over the world come together to create Free Software for a free internet. This is made possible by the support, hard work and enthusiasm of thousands of people, including those who create and use Nextcloud software.
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+Our code of conduct offers some guidance to ensure Nextcloud participants can cooperate effectively in a positive and inspiring atmosphere, and to explain how together we can strengthen and support each other.
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+The Code of Conduct is shared by all contributors and users who engage with the Nextcloud team and its community services. It presents a summary of the shared values and “common sense” thinking in our community.
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+You can find our full code of conduct on our website: https://nextcloud.com/code-of-conduct/
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+Please, keep our CoC in mind when you contribute! That way, everyone can be a part of our community in a productive, positive, creative and fun way.