docs: helpful step on getting started guide

documentation/tutorials/getting-started-with-ash-authentication-phoenix.md

@@ -316,6 +316,15 @@ defmodule Example.Accounts do
 end
 ```
 
+### Add to config
+
+Although mentioned in a step at the top, a common mistake here is not to add the new api into your `ash_apis` config in `config/config.exs`. It should look like this:
+
+```elixir
+config :example,
+  ash_apis: [..., Example.Accounts]
+```
+
 ### Create and Migration
 
 Now is a good time to create the database and run the migrations. You have to use specific `ash_postgres` mix tasks for that: