Add 'workers' section into configuration doc
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
stackforge/watcher,openstack/watcher,stackforge/watcher,openstack/watcher