Document how to use harbormaster.sendmessage to report lint and unit results

Summary: Fixes T7419. This doesn't really do anything, just adds documentation.

Test Plan:

• Read the documentation:

• Created a build plan which makes an HTTP request to example.com and waits for a result.
• Ran that build plan manually.
• Called harbormaster.sendmessage manually with the example lint/unit values to provide a result.
• Saw the results report correctly and the message ("fail") process as expected:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7419

Differential Revision: https://secure.phabricator.com/D13789