Add Scan Workflow Diagram

Also added description for all steps

Updated Workflow diagram, added more details in description

Change-Id: I8ade7c130cd03acdd348b88e593fc8d054a845b2
Reviewed-on: http://review.couchbase.org/34385
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>
Reviewed-by: Sriram Melkote <siri@couchbase.com>