Add documentation of log levels (#93)

* Add documentation of log levels

Signed-off-by: Tully Foote <[email protected]>

rcl_interfaces/msg/Log.msg

@@ -1,5 +1,14 @@
 ##
 ## Severity level constants
+## 
+## These logging levels follow the Python Standard
+## https://docs.python.org/3/library/logging.html#logging-levels
+## And are implemented in rcutils as well
+## https://github.com/ros2/rcutils/blob/35f29850064e0c33a4063cbc947ebbfeada11dba/include/rcutils/logging.h#L164-L172
+## This leaves space for other standard logging levels to be inserted in the middle in the future,
+## as well as custom user defined levels.
+## Since there are several other logging enumeration standard for different implementations,
+## other logging implementations may need to provide level mappings to match their internal implementations.
 ##
 
 # Debug is for pedantic information, which is useful when debugging issues.