bring back the Code of Conduct

imported from http://web.archive.org/web/20131011085000/http://owncloud.org/about/community-code-of-conduct/

@danimo @karlitschek @jospoortvliet @tomneedham

@tomneedham can we create some kind of short url on owncloud.org which will point to this page on doc.owncloud.org? (once merged of course)