Create developer-notes.md and add ack/nack explanations

Following up on ACK/NACK discussion on the mailing list. Couldn't figure out where the best place to add documentation for pull request terms would be. Decided that creating a developer notes file and combining the existing notes/guidelines could be beneficial.

Moves coding guidelines and development tips/tricks into a single file.
Also adds a section explaining ack/nack.
This is IMHO also true for qWarning() currently.