Write documentation for setting up CI on 3rd party repositories

[rikkolovescats]

continuing from #156 (comment).

we have a ci here in the main plugin manager repo which checks and corrects metadata whenever a PR is opened, makes things convinent by not having to fill in metadata manually stuff each time; docs on how to setup such a CI is currently missing for people who want to create their own third party plugin sources.

maybe(?) this should ideally be handled in https://github.com/bombsquad-community/sample-plugin-source

[kingsamurai123]

Hello rikko,
doesn't this be resolved, if they use the same CI workflow on their repo while they are building the plugin?