Module page docummentation

Problem/Motivation

This module's main page includes a section that directs developers on how to implement their own checks by using hook_prod_check_alter(), however, this doesn't seem to be the method used for the current 2.x version of the module, and presumably was the method used for the older D7 versions of the module. The current module appears to implement a plugin system, so I would assume that developers wanting to implement their own checks should create plugin classes in the Drupal\{my_module}\Plugin\ProdCheck\{CheckType} namespace, extending the ProdCheckBase class. Is that correct? Should we update the main page documentation to clarify this?