Add 'workers' section into configuration doc

Workers has been introduced into Decision Engine and Applier.
We can now tune ithe number of workers (threads) used by the
Decision Engine and the Applier services, by updating the Watcher
configuration file.

Change-Id: I81523666e4373e7b1dd59b36daf19067fd61b48f
Closes-Bug: #1528132

doc/source/deploy/configuration.rst

@@ -318,3 +318,33 @@ pick any database system you prefer.
 The original implementation has been based on MongoDB but you can create your
 own storage driver using whatever technology you want.
 For more information : https://wiki.openstack.org/wiki/Gnocchi
+
+
+Workers
+=======
+
+You can define a number of workers for the Decision Engine and the Applier.
+
+If you want to create and run more audits simultaneously, you have to raise
+the number of workers used by the Decision Engine::
+
+    [watcher_decision_engine]
+
+    ...
+
+    # The maximum number of threads that can be used to execute strategies
+    # (integer value)
+    #max_workers = 2
+
+
+If you want to execute simultaneously more recommended action plans, you
+have to raise the number of workers used by the Applier::
+
+    [watcher_applier]
+
+    ...
+
+    # Number of workers for applier, default value is 1. (integer value)
+    # Minimum value: 1
+    #workers = 1
+