DOC: Added `existing_model` to general user documentation.

src/doc/yaml_interface.rst

@@ -125,7 +125,14 @@ You can use the yaml parser directly in python as follows:
 
 This creates a DataModel object containing all entities defined in the yaml file.
 
-You can then use the functions from caosadvancedtools.models.data_model.DataModel to synchronize
+If the parsed data model shall be appended to a pre-exsting data model, the optional
+``extisting_model`` can be used:
+
+.. code-block:: python
+
+   new_model = parser.parse_model_from_yaml("model.yml", existing_model=old_model)
+
+You can now use the functions from ``DataModel`` to synchronize
 the model with a CaosDB instance, e.g.:
 
 .. code-block:: python