clarify discovery service information
clarify discovery service information

 - specify that we need to use a discovery service
 - indicate which specific discovery service we are using for this example
 - describe the token discovery service as hosted at Docker Hub
 - link to other available discovery services in the docs

Signed-off-by: Morgan Bauer <9014cf436b229fe76f032246dafdb62784667fb6@us.ibm.com>
(cherry picked from commit 6d8fea06640d00c1a6a711d9313872031dee94db)

beni55/swarm-1,beni55/swarm-1