Clarify project scope

I've spoken with the maintainer of Entity Reference Exposed Filters and we've taken a look at the differences between both modules. The conclusion is that they are complementary because Views Entity Reference Filter uses the Drupal\views\Plugin\views\filter\InOperator base class and Entity Reference Exposed Filters uses the Drupal\views\Plugin\views\filter\ManyToOne base class, each with their own specific use case. That said, we also concluded that both modules are solving a highly similar issue and are probably best combined to prevent duplication of work.

Both of us were also aware of the core issue, but agreed that this functionality needs to stay available in contrib for the foreseeable future, given the fact that core issues can take very long to land.

Given the fact that both modules have similar usage statistics, but Views Entity Reference Filter has cleaner code and test coverage, we've decided that it's best to merge Entity Reference Exposed Filters into Views Entity Reference Filter and provide an upgrade path for existing users.

#3062753: [PLAN] Merge Entity Reference Exposed Filters was created to coordinate this effort