Document GitHub token usage

Provide brief summary of [creating a GitHub token](https://docs.github.com/en/authentication/keeping-your-account-and-data-secure/creating-a-personal-access-token) and [configuring git to use the token](https://git-scm.com/docs/gitcredentials) using  [`git-credential-store`](https://git-scm.com/docs/git-credential-store) or [`git-credential-cache`](https://git-scm.com/docs/git-credential-cache).