README: Add sections about features/project structure, getting started, and development/contributing

• Section "Features" or "Project Structure", explaining what functionality is implemented and in which Python package it lives
• Section "Getting started", giving a short introduction on how to install PyI40AAS (from Git or PyPI) and pointing to the tutorials and examples
• Section "Development", explaining how to set up a development environment and running tests (incl. )
• Section "Contributing", explaining our coding guidelines (how to run mypy and pycodestyle), workflows (GitLab PRs) and pointing to issues