Make CONTRIBUTING docs

[ggbaro]

• Code of conduct
• Docs on how to make docs
  • See: About Documentation #18 (comment)
• List of good first issues
• Finish this list of contents to include in CONTRIBUTING

[LFK01]

A start point for contributions could be comments on the code.

For example I don't understand what is the code executed when the CLI command is invoked, I'm pretty sure I checked all the python files (mostly _base.py and airtable.py) and I haven't found any ArgumentParser or sys.argv call to parse CLI parameters.

Another idea for the contributing could be a little explanation on how the code is executed, from there I'll try to help as much as possible. Pardon my juniority :)