Clarify the module's behavior with SearchAPI filters

Problem/Motivation

I spent quite a lot of time walking through the code and learning how the entire module was coded, only to be stumped why the filter would only work on a small subset of possible values.

Proposed resolution

Add clarification to the README.md file that informs site builders of the possible issues surrounding the SearchAPI fields. The issue has already been addressed and deemed not a bug, but does cause a seriously confusing situation for site builders.

https://www.drupal.org/project/selective_better_exposed_filters/issues/3... 🐛 SearchAPI setItemsPerPage Closed: works as designed

User interface changes

None

API changes

None

Data model changes

None