Attempt at mermaid for model description

model/model.md

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+# PairGoth model
+
+## Entity Relationship Diagram
+
+```mermaid
+erDiagram
+
+  %% entities
+
+Tournament {
+    name
+    shortName
+    startDate
+    endDate
+    location
+    isOnline
+  }
+
+  Round {
+    number
+  }
+
+  Game {
+    black
+    white
+    result
+  }
+
+  Player {
+    lastname
+    firstname
+    country
+    club
+  }
+
+  Ladder {
+    name
+    lastUpdated
+  }
+  
+  %% relationships
+
+  Tournament ||--|{ Round
+  Round ||--|{ Game
+  Game |}--|| Player : black
+  Game |}--|| Player : white
+  Player }o--o{ Ladder : rating
+  
+```
+