Update documentation about pygac

[carloshorn]

Nice, we got a hacktoberfest tag.
I could also provide some instructions on the calibration coefficient creation and the json layout.
Does pytroll have a style guide for the documentation? Otherwise, I would just try to write it as similar as possible to the existing docs.

[sfinkens]

Yeah, but now it's over 😱

Nevertheless, sounds great! I think as long as it's not too much text, we can stick to the README. But at some point we should probably change to sphinx. I'm not aware of a style guide for docs. But the existing pygac docs, plus maybe satpy & pyresample are probably a good reference.