Add custom type example file

examples/types/custom-01.ex

@@ -0,0 +1,30 @@
+defmodule FilledString do
+  use Drops.Type, {:string, [:filled?]}
+end
+
+defmodule UserContract do
+  use Drops.Contract
+
+  schema do
+    %{
+      required(:name) => FilledString
+    }
+  end
+end
+
+UserContract.conform(%{name: "Jane Doe"})
+# {:ok, %{name: "Jane Doe"}}
+
+{:error, errors} = UserContract.conform(%{name: 1})
+Enum.map(errors, &to_string/1)
+# ["name must be a string"]
+
+{:error, errors} = UserContract.conform(%{name: ""})
+Enum.map(errors, &to_string/1)
+# ["name must be filled"]
+
+[%{type: type}]= UserContract.schema().keys
+# %FilledString{
+#   primitive: :string,
+#   constraints: {:and, [predicate: {:type?, :string}, predicate: {:filled?, []}]}
+# }