more info on workers (#151)

Author: mehdigmira

docs/workers.md

@@ -6,6 +6,9 @@ It is started with a list of queues to listen to, in a specific order.
 
 It can be started with concurrency options (multiple processes and / or multiple greenlets). We call this whole group a single 'worker' even though it is able to dequeue multiple jobs in parallel.
 
+If a worker is started with concurrency options, it will poll for waiting jobs and dispatch them to its related processes/greenlets.
+For example, if we decide to use the greenlets option, under the hood, the worker will be one python process that has a pool of greenlets which will be in charge of actually running tasks.
+
 
 ## Statuses