Look into badge for our repo from OpenSSF

[LeandraH]

Is your feature request related to a problem? Please describe.
As Sven pointed out in https://chat.tuerantuer.org/digitalfabrik/pl/n93dgppw5jgojqbf4r1kit1rcw , there is a badge from Open Source Security Foundation about which best practices a repo follows. Maybe we want to implement that as well in our repo, to encourage us to implement / stick to best practices, and to show people who are interested in contributing that we care about security.

Here is the report for the CMS: https://securityscorecards.dev/viewer/?uri=github.com/digitalfabrik/integreat-cms

And a link to the best practices from the OpenSSF: https://www.bestpractices.dev/de

Describe the solution you'd like
Have a shiny new badge in our repo that says that we do a good job on security.

Describe alternatives you've considered
We could check on whether we follow the best practices without adding a badge. Or we could ignore this altogether.