docs(github-actions): add triggering semantic-release on private repositories

[jimmiebee]

This PR adds a docs regarding sample workflow trigger for private repositories.

[travi]

could you clarify the use case that you are trying to capture with this contribution? what do you mean by private in this case?

setting "private": true in the package.json prevents publishing a package altogether. in a way, that property is named poorly because it truly means something closer to "this project is not a package". if you are targeting projects that are not packages with this contribution, i dont think that is clear with this iteration.