introducing pull request review process


basically imported from http://web.archive.org/web/20131013081848/http://owncloud.org/code-reviews-on-github/ and adjusted

@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)