[AIRFLOW-31][AIRFLOW-200] Add note to updating.md

AIRFLOW-31 and AIRFLOW-200 deprecated the old important mechanism and should be noted in UPDATING.md

Closes apache#1643 from jlowin/patch-1

UPDATING.md

@@ -20,8 +20,13 @@ dags_are_paused_at_creation = False
 ### Deprecated Features
 These features are marked for deprecation. They may still work (and raise a `DeprecationWarning`), but are no longer supported and will be removed entirely in Airflow 2.0
 
-#### Operators no longer accept arbitrary arguments
-Previously, `Operator.__init__()` accepted any arguments (either positional `*args` or keyword `**kwargs`) without complaint. Now, invalid arguments will be rejected.
+- Hooks and operators must be imported from their respective submodules
+
+  `airflow.operators.PigOperator` is no longer supported; `from airflow.operators.pig_operator import PigOperator` is. (AIRFLOW-31, AIRFLOW-200)
+
+- Operators no longer accept arbitrary arguments
+
+  Previously, `Operator.__init__()` accepted any arguments (either positional `*args` or keyword `**kwargs`) without complaint. Now, invalid arguments will be rejected. (https://github.com/apache/incubator-airflow/pull/1285)
 
 ## Airflow 1.7.1.2