Add efx (#4886)

README.md

@@ -1506,6 +1506,7 @@ There are [other sites with curated lists of elixir packages](#other-awesome-lis
 * [dummy](https://github.com/Vesuvium/dummy) - An Elixir mocking library that makes sense by exposing meck in the right way.
 * [ecto_it](https://github.com/xerions/ecto_it) - Ecto plugin with default configuration for repos for testing different ecto plugins with databases.
 * [efrisby](https://github.com/FabioBatSilva/efrisby) - A REST API testing framework for erlang.
+* [efx](https://github.com/bravobike/efx) - A library to declaratively write asynchronously testable effects.
 * [elixir-auto-test](https://github.com/joaothallis/elixir-auto-test) - Run test when file is saved using inotify-tools.
 * [espec](https://github.com/antonmi/espec) - BDD test framework for Elixir inspired by RSpec.
 * [espec_phoenix](https://github.com/antonmi/espec_phoenix) - ESpec for Phoenix web framework.