Document "when and what to contribute"

[6543]

#2152 first started with development guidelines ... which I first document at #3184

But there are still useful thoughts in the discussion, not yet documented!

Also another topic that was talked about, was when it should be an addon and when it should be contributed into woodpecker itself, also document that :)

[qwerty287]

For information about what we discussed: #2152 (reply in thread) and the comments below are a good summary