Add basic information to "how to write custom recorder" docs

We had it on the todo list, but since #1274 (closed) raises a question about that I prefered to add already something to the docs. Yes, it is very minimal infomration, but may be already usefull. @sardana-org/integrators could you check that? Also @stanislaw55 it would be great if you could review it and comment (or maybe even complete:)) based on your experience in wrting recorders...

Thanks to all!