[FEATURE] Docs: Explain how to obey the AGPLv3 license

This project uses the AGPLv3 license. If an admin decides
to make some adaptions to the source code, and run a service with
these changes incorporated, the admin is required to offer
the servers users a download of the source code of his version.

Describe in the README how to do that conveniently with the ToS page.

Closes #95