adding secret vars as demo

[harrisonravazzolo]

Got a request to add an example of secret vars to the starter repo, added necessary files and update to the README.

[getvictor]

The repo won't work when cloned unless they add a repo secret, right? And what about GitLab -- need instructions for that. Maybe we can keep this example on a branch or we can instead document how to explicitly enable this example? Like we comment out the script in the yaml file by default.

[harrisonravazzolo]

I tested it off a clone and it didn't throw an error but I will try it again just to double-check. But I also need to update the readme for GitLab -- good catch.

[noahtalerman]

@harrisonravazzolo still important? If not maybe we close this PR. Up to you.