Detail the Usage of Private Repositories for GitHub Actions

[gcw-it]

Fixes #306

[gcw-it]

• I removed the non-breaking chars from the code section.
• I left the clause, restricting the access to the actions to the same org/user, in for now. If you still think, it's better to remove it, I'll do so.

[gsmet]

Thanks! Will merge this one and the other one and prepare a release!