add code of conduct that is a lightly edited version of Debian's
At long last, I'm proposing that F-Droid adopt Debian's code of conduct as our own. I copied the text from there, changed Debian to F-Droid, and changed the contact section to make sense for F-Droid.
I'm pinging everyone to make sure there is no objections @fdroid
Edited by Hans-Christoph Steiner