Document how to do periodic builds using Packit CLI

[nforro]

Example of a GitHub action that runs packit build in-copr every day:
https://github.com/filippor/FreeCAD/blob/4bd71e8cca1e5adde019172b1e451900b6edbfd1/.github/workflows/fedora-daily.yml

[filippor]

updated to get history and tag for changelog and version
https://github.com/filippor/FreeCAD/blob/5768bbe24019c59f8f514c661c4f3330369eea0b/.github/workflows/fedora-daily.yml