docs(airflow): add docs on custom operators (datahub-project#7913)

Co-authored-by: John Joyce <[email protected]>

docs/lineage/airflow.md

@@ -69,14 +69,41 @@ lazy_load_plugins = False
 
 ### How to validate installation
 
-1. Go and check in Airflow at Admin -> Plugins menu if you can see the Datahub plugin
+1. Go and check in Airflow at Admin -> Plugins menu if you can see the DataHub plugin
 2. Run an Airflow DAG. In the task logs, you should see Datahub related log messages like:
 
 ```
-Emitting Datahub ...
+Emitting DataHub ...
 ```
 
-## Using Datahub's Airflow lineage backend (deprecated)
+### Emitting lineage via a custom operator to the Airflow Plugin
+
+If you have created a custom Airflow operator [docs](https://airflow.apache.org/docs/apache-airflow/stable/howto/custom-operator.html) that inherits from the BaseOperator class,
+when overriding the `execute` function, set inlets and outlets via `context['ti'].task.inlets` and `context['ti'].task.outlets`.
+The DataHub Airflow plugin will then pick up those inlets and outlets after the task runs. 
+
+
+
+```python
+class DbtOperator(BaseOperator):
+    ...
+
+    def execute(self, context):
+        # do something
+        inlets, outlets = self._get_lineage()
+        # inlets/outlets are lists of either datahub_provider.entities.Dataset or datahub_provider.entities.Urn
+        context['ti'].task.inlets = self.inlets
+        context['ti'].task.outlets = self.outlets
+
+    def _get_lineage(self):
+        # Do some processing to get inlets/outlets
+        
+        return inlets, outlets 
+```
+
+If you override the `pre_execute` and `post_execute` function, ensure they include the `@prepare_lineage` and `@apply_lineage` decorators respectively. [source](https://airflow.apache.org/docs/apache-airflow/stable/administration-and-deployment/lineage.html#lineage)
+
+## Using DataHub's Airflow lineage backend (deprecated)
 
 :::caution
 
@@ -145,6 +172,7 @@ Take a look at this sample DAG:
 
 In order to use this example, you must first configure the Datahub hook. Like in ingestion, we support a Datahub REST hook and a Kafka-based hook. See step 1 above for details.
 
+
 ## Debugging
 
 ### Incorrect URLs