Add a comment describing durability.

Signed-off-by: Chris Lalancette <clalancette@osrfoundation.org>

Author: clalancette

image_tools_py/src/image_qos_profile.py

@@ -46,6 +46,10 @@ def create_qos_profile(depth, reliability_policy, keep):
     else:
         raise ValueError("Invalid keep policy (must be 0 for keep last or 1 for keep all")
 
+    # The durability policy determines what happens to samples that were sent before
+    # a subscribe joined the network.
+    # VOLATILE - Past samples ignored
+    # TRANSIENT_LOCAL - Past samples are sent to the subscriber
     qos_profile.durability = QoSDurabilityPolicy.RMW_QOS_POLICY_DURABILITY_VOLATILE
 
     return qos_profile