Document the GitHub events we subscribe to and how they are used

[joshuagl]

Describe the solution you'd like
We subscribe to various GitHub events on the installed repositories in order to know when we should be performing an activity.

Whilst it is easy enough to determine which events and actions trigger processing within Precaution by reading the code it's not always obvious why those events and actions are important and how the flow of events/actions directs Precaution's processing.

We should document the event processing flow within Precaution, events we subscribe to and actions we handle within those events, so that new developers can understand the flow and existing developers have a reference when working on Precaution.

Additional context

• GitHub Event Types & Payloads documentation
• PR introducing handling of PullRequestEvent's synchronize action Fix for "across the forks" PRs problems #113
• PR removing handling of CheckSuiteEvent's requested action Remove spurious handling of CheckSuiteEvent's requested action #199