doc: Expand README.md

The readme file should include the following sections:

• Features – list-style but comprehensible description of the provided features of this project
• Getting started –
  • Example – Short code block (~25 loc) demonstrating the main features of the framework. May be part of the Getting Started (no individual heading)
  • References to further resources (examples)
  • Project Structure – Short overview over the included packages and modules (only those wihtin the aas package). This may be merged somehow with the Features section
• Contributing
  • Information about our codestyle and testing
  • Information about how to contribute Code/patches