Update documentation

[sfinkens]

Some ideas:

• Command line and library usage
• Explain config file contents
• Include API of reader classes (so that the satpy reader can refer to this for available reader_kwargs)
• Installation instructions
  • There's no need for using prefix with virtualenv or conda. Just pip install pygac for the latest release or pip install git+... for the latest development version
  • Development installation instructions (git clone && pip install -e .) would be nice, too

[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.