docs: update readme to include steps to test and confirm changes for a PR

[keevan]

This will be helpful for the reviewer, as well as the person creating the change.

This allows the changes to be fully tested and confirmed working.

How:

The options introduced to assist with this process is internal_workflow_branch (https://github.com/catalyst/catalyst-moodle-workflows/blob/main/.github/workflows/ci.yml#L7-L10)

So in the plugin branch, you would create or update the ci.yml file (as per current docs) in a new test branch, and set the with option internal_workflow_branch to the branch being PR'd in this repository.

It will then run the actions using the new branch, instead of the (default) main branch.

The person making the changes will then link these tests / references in their PR as part of their changes