Document how to use Ecto.ULID as prolific primary key

README.md

@@ -72,15 +72,32 @@ create table(:events, primary_key: false) do
 end
 ```
 
+Alternatively, if you plan to use ULID as the primary key type for all of your tables, you can set
+`migration_primary_key` when configuring your `Repo`:
+
+```elixir
+config :my_app, MyApp.Repo, migration_primary_key: [name: :id, type: :binary_id]
+```
+
+and then you *do not* need to specify the `id` column in your migrations:
+
+```elixir
+create table(:events) do
+  # more columns ...
+end
+```
+
 ### Schema
 
-When defining a model's schema, use `Ecto.ULID` as the `@primary_key`:
+When defining a model's schema, use `Ecto.ULID` as the `@primary_key` or `@foreign_key_type` as
+appropriate for your schema. Here's an example of using both:
 
 ```elixir
 defmodule MyApp.Event do
   use Ecto.Schema
 
   @primary_key {:id, Ecto.ULID, autogenerate: false}
+  @foreign_key_type Ecto.ULID
   schema "events" do
     # more columns ...
   end