Basic documentation for Migrate Source YAML

Problem/Motivation

It would be great with some very basic info on how to use this module.

Steps to reproduce

Want to use the module, but not sure how to ...

Proposed resolution

Document the basic steps and/or components to use the Migrate Source YAML module. Probably all that is needed is this?

• The syntax in the migration file
• The structure of a YAML source file

Remaining tasks

Document the basic set up in a README file, or a new page under https://www.drupal.org/docs/contributed-modules/ →

User interface changes

API changes

Data model changes