added Workflow system in introduction, both to the diagram and the prose

index.html

@@ -464,6 +464,17 @@ <h3>Storage Services</h3>
             (Verifier) &bull; Storage Service (Holder)
           </strong>
         </p>
+      <section>
+        <h3>Workflow Service</h3>
+        <p>
+The Workflow Service provides a way for coordinators to automate specific
+interactions for specific users. Each role (Holder, Issuer, and Verifier)
+can run their own Workflow Service to create and manage exchanges that
+realize particular workflows. Administrators configure the workflow system
+to support particular flows. Then, when the business rules justify it,
+coordinators create exchanges at their Workflow Service and give authorized
+access to those exchanges to any party. 
+        </p>
         <p>
 Each actor in the system is expected to store their own verifiable
 credentials, as needed. Several known implementations use secure data