up README with sample

README.md

@@ -13,6 +13,25 @@ def deps do
 end
 ```
 
+## Use
+
+```elixir
+defmodule MyApp.User do
+  use MyApp.Web, :model
+
+  @primary_key {:id, :binary_id, autogenerate: true}
+  @derive {Phoenix.Param, key: :id}
+
+  schema "users" do
+    field :email, :string
+    field :provider, Ecto.Atom  # :github => 'github' in DB
+    field :uid, :string
+    field :token, :string
+    ...
+  end
+end
+```
+
 Documentation can be generated with [ExDoc](https://github.com/elixir-lang/ex_doc)
 and published on [HexDocs](https://hexdocs.pm). Once published, the docs can
 be found at [https://hexdocs.pm/ecto_atom](https://hexdocs.pm/ecto_atom).