initial sequence diagram for verified contact flow

references #29.

Kept the sequence diagram close to the current numbering in the new.rst doc.
Would probably good to have different numbers like 3.a) 3.b)
for the separate parts of all the steps.

Also not using the resulting diagram in the pdf yet.
Diagram is fairly big and will probably result in a full page.
This should be fine as it allows comparing the instructions
and the diagram side by side.

images/contact.seq

@@ -0,0 +1,24 @@
+# This is a sequence diagram for the dkim section
+# I used https://bramp.github.io/js-sequence-diagrams/
+# to render it to svg and then save it in images folder
+
+participant Alice as Ua
+participant Bob as Ub
+
+Note over Ua: 1. track bootstrap per INVITENUMBER
+Ua --> Ub: 1. bootstrap code
+Note over Ub: 2.a) check for exising key
+Ub ->> Ua: 2.b) vc-request message with INVITENUMBER
+Note over Ua: 3. look up bootstrap by INVITENUMBER
+Note over Ua: 3. abort if invite expired
+Note over Ua: 3. process AC header
+Ua -> Ub: 3. vc-auth-required message with AC header
+Note over Ub: 4.a) abort if key does not match FP from bootstrap
+Ub -> Ua: 4.b) vc-request-with-auth with Bob_FP and AUTH
+Note over Ua: 5. verify AUTH and key
+Note over Ua: 5. alert user on failure and abort
+Note over Ua: 6. signal success to user
+Ua -> Ub: 6. vc-contact-confirm message
+Note over Ua: 6. clear bootstrap data for INVITECODE
+Note over Ub: 7. signal success to user
+