Provide basic documentation

Created by: elbenfreund

We should provide at least some basic documentation which should cover

• User guidance
  • Dependencies
• Development notes
  • Styleguide
  • Contribution guidelines
  • Notes on how to document code / contributions
  • Explanations on why we did things a certain way
  • Highlighting and explain genuine gnome-shell specific parts of the codebase
  • General information (where relevant) on how we interact with the general gnome-shell infrastructure and where to find more resources on that (this is especially important as documentation is scars)

Code documentation should follow JSDoc syntax rules to make it as easy as possible to autogenerate module documentation at a later stage.

The actual infrastructure is provided by #119 (closed) so this issue is blocked by it.