Spinning off the discussion from rpm-software-management/createrepo_c#75 (comment)

I am investigating whether we can use appstream-generator for Copr instead of appstream-builder. But even after reading all the documentation and man pages and output of --help I still do not know how to use appstream-generator.

My questions are:

1. What will generate the asgen config? Because I cannot write it for every project in Copr.
2. What I need to run on the first run in the directory?
3. What I need to run on every consecutive run in the directory? Should I keep some files/dirs to improve the performance?