Rename Trust Bundles to Registries (closes #15)
Update DocumentReference search params.  #14 #10 #13
Documented Single-Provider discovery.  (Closes #5)
Document strategy for linking Registry --> Trust Bundle File (closes #16)
Simplified DocumentReference 'type' and added 'standard' parameter
to distinguish among CCDA/CCD 1.0/CCR.  Addresses #9 and #13
Document required fields for DocumentReference (Closes #10)