Fix #391: put user creation logic in a importable function

docs/usage/helpers.md

@@ -0,0 +1,22 @@
+# Helpers
+
+## Create user
+
+**FastAPI Users** provides a helper function to easily create a user programmatically. They are available from your `FastAPIUsers` instance.
+
+```py
+regular_user = await fastapi_users.create_user(
+    UserCreate(
+        email="king.arthur@camelot.bt",
+        password="guinevere",
+    )
+)
+
+superuser = await fastapi_users.create_user(
+    UserCreate(
+        email="king.arthur@camelot.bt",
+        password="guinevere",
+        is_superuser=True,
+    )
+)
+```