chore: add note to CONTRIBUTING.md about how to run tests against Connect #728

We removed the test-connect jobs (#727) from this repository and adjusted the functionality of the rsconnect-python-tests-at-night in Connect so that they can be run against a specific branch of rsconnect-python via workflow dispatch.

We need to document that these tests should be triggered on the Connect side for each new PR in rsconnect-python and give instructions for how to do so.