Add documentation for using this module

This tutorial has a thorough instructions. It should be added as an External Documentation link:

https://ws.agency/blog/indexing-flags-drupal-8-flag-search-api-module

A quick summary:

Flag indexing is available as a Processor in the Search index settings. To configure flag indexing, check the Flag indexing box on the Processors tab of your search index. Then re-index content. Now the Flag data will be accessible in views.