Missing documentation for many configuration values.

[saiichihashimoto]

Often, an issue arises like this one where someone asks for support of a new config value, but it turns out it's already there. Is this something we can fix as an issue across the board? I'm wondering which of these is true:

1. probot/settings is being updated to support these values but we are forgetting to update the README. If so, totally understandable, not much to do here.
2. These config values are actually coming from an API so any new values are actually natively supported. If so, we should reference that API in the documentation, so we can know that it's an ongoing list, and have a link to that list.