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

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information