Docs: Add validators to dynamic model (#1963)

* Add dynamic validators to doc

* Update models_dynamic_validators.py

* Update models_dynamic_validators.py

* Adding example (success and error)

* Update models_dynamic_validators.py

docs/examples/models_dynamic_validators.py

@@ -0,0 +1,26 @@
+from pydantic import create_model, ValidationError, validator
+
+
+def username_alphanumeric(cls, v):
+    assert v.isalnum(), 'must be alphanumeric'
+    return v
+
+
+validators = {
+    'username_validator':
+    validator('username')(username_alphanumeric)
+}
+
+UserModel = create_model(
+    'UserModel',
+    username=(str, ...),
+    __validators__=validators
+)
+
+user = UserModel(username='scolvin')
+print(user)
+
+try:
+    UserModel(username='scolvi%n')
+except ValidationError as e:
+    print(e)