Fix #396: add password validation example

docs/configuration/model.md

@@ -40,6 +40,23 @@ class UserDB(User, models.BaseUserDB):
 
 You can of course add you own properties there to fit to your needs!
 
+## Password validation
+
+**FastAPI Users** doesn't provide a default password validation, but you can implement it easily with a [Pydantic validator](https://pydantic-docs.helpmanual.io/usage/validators/) on the `UserCreate` class. Here is a simple example to check if the password is at least six characters long:
+
+```py
+from fastapi_users import models
+from pydantic import validator
+
+
+class UserCreate(models.BaseUserCreate):
+    @validator('password')
+    def valid_password(cls, v: str):
+        if len(v) < 6:
+            raise ValueError('Password should be at least 6 characters')
+        return v
+```
+
 ## Next steps
 
 Depending on your database backend, database configuration will differ a bit.