update README and add CONTRIBUTING.md
@obfusk @jspricke @uniqx @linsui @FestplattenSchnitzel @brandsimon @zx2c4 @licaon-kter @TheLastProject ping you as recent contributors.
Edited by Hans-Christoph Steiner
@obfusk @jspricke @uniqx @linsui @FestplattenSchnitzel @brandsimon @zx2c4 @licaon-kter @TheLastProject ping you as recent contributors.