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.
I nit is simply a small comment. It comes from 'nitpicking' which is English, which means 'to be excessively concerned with or critical of inconsequential details'. It's a kind of self-criticizing at time same time. 'I like it slightly better like this but it isn't really important'...
It's a kind of silly colloquialism and doesn't belong in this 'formal' list.