Feature/12 add uml mermaid diagram (#15)

* initial pass at something

* incorrect association

* chaser

* initial pass

* aligning the comments

docs/webapi-to-backend-ballot-UML.md

@@ -0,0 +1,41 @@
+
+```mermaid
+classDiagram
+  Choice *-- Selection
+  Ballot *-- Contest
+  Contest *-- Selection
+  Choice *-- Ticket
+
+  class Choice {
+    +String name
+    +String party
+    +Dict ticket
+    +String choice_type
+  }
+
+  class Selection {
+    +Int index
+    +String name
+  }
+
+class Ticket {
+    +String name
+    +String party
+}
+
+  class Contest {
+    +List choices          # order is important
+    +String vote_variation # RCV, plurality
+    +String uid            # unique to election only
+    +Float win_threshold   # default = 0.5
+    +Int votes_allowed     # defines overvote
+    +String write_in       # unimplemented
+    +List selection        # index + name
+  }
+
+  class Ballot {
+    +List active_ggos      # order is important
+    +Dict contests         # ordered by active_ggos
+    +String ballot_status  # blank or cast
+  }
+```