[doc] update the README file

to explain a bit the example celeryconfig.py file.

README

@@ -22,6 +22,13 @@ celeryconfig.py example::
     CELERY_ACCEPT_CONTENT = ['json', 'msgpack', 'yaml']
 
 
+In this configuration example, the ``cw_celerytask_helpers`` in
+``CELERY_IMPORTS`` is required to have logging data (in the task) sent
+back to the Cubicweb instance via Redis. The
+``CUBICWEB_CELERYTASK_REDIS_URL`` is the Redis endpoint used for this
+logging handling mechanism.
+
+    
 Start a worker::
 
     # running cubicweb tasks (celeryconfig.py will be imported from your instance config directory)
@@ -44,7 +51,6 @@ variable (it must be an importable python module).
 Running tasks
 -------------
 
-
 Create a task:
 
 .. code-block:: python