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?

added Help wanted label

mentioned in merge request !20 (merged)

marked the checklist item List of features, maybe as completed

marked the checklist item List of features, maybe as incomplete

marked the checklist item Explanation on how to run production as completed

marked the checklist item Maybe also explanation on how to run it without Docker, e.g. in the pool as completed

marked the checklist item Explanation on how to run the dev setup with Docker as completed

Yay, 3/7 Checkboxes!

marked the checklist item List of features, maybe as completed

marked the checklist item General explanation about the purpose of Portal as completed

5/7, gotta deal with the other boxes another time. Until we meet again, Portal (hopefully soon.)