Add helpful README

The README currently is very bare. We should add more helpful content to it, to make it e.g. like https://gitlab.com/schwalbe-hd/schwalbe/-/blob/master/README.md?ref_type=heads

What is missing:

• General explanation about the purpose of Portal
• List of features, maybe
• Explanation on how to run the dev setup with Docker
• Maybe also explanation on how to run it without Docker, e.g. in the pool
• Explanation for the proxy-ldap.sh script (check with @moser)
• Explanation on how to run production
• Tips on how to get started

Any other thoughts on what could be added?