CI: Document required secrets to run CI in a fork

[andrewthetechie]

To run the CI jobs in the fork, you will need to setup a secret THIS_PAT that contains a PAT that has read access to your repo's settings. It is used by the action integration test.

[actuarysailor]

@andrewthetechie - the new change you made to coalesce this_pat with GitHub.token fixes this.