Ensure official plugin settings are documented

[kdmccormick]

Context

This is follow-up from openedx-unsupported/wg-developer-experience#6

Examples

The MFE plugin does a good job of describing its settings and how one can use them to customize the plugin: https://github.com/overhangio/tutor-mfe/

So does the Notes plugin: https://github.com/overhangio/tutor-notes

Acceptance Criteria

1. Add a README section to plugin template repo inviting plugin authors to document the settings that they have defined
2. For each official Tutor plugin, either:

• Document any config settings defined by plugin in its README, or
• Create an issue in the plugin repo for documenting those settings.