Document -Dmaven.gitcommitid.skip=true in FAQ and elsewhere

[elharo]

Describe the idea (required)

Had to find about this on stack overflow, and didn't find it on my first goiogle search. Add it to the FAQ

Tell us about the expected behaviour (required)

It's doucmented

Context (optional)

Useful when you're building from non git source; e.g. from a tarball.
In my case code is dual hosted in both git and non-git source repos.

[TheSnoozer]

Thanks for the suggestion!
Done in https://github.com/git-commit-id/git-commit-id-maven-plugin/blob/master/docs/faq.md#plugin-options-and-configurations