Initial documentation for ER diagrams

docs/entityRelationshipDiagram.md

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+# Entity Relationship Diagrams
+
+> An entity–relationship model (or ER model) describes interrelated things of interest in a specific domain of knowledge. A basic ER model is composed of entity types (which classify the things of interest) and specifies relationships that can exist between entities (instances of those entity types). Wikipedia.
+
+Mermaid can render ER diagrams
+```
+erDiagram
+    CUSTOMER !-?< ORDER : places
+    ORDER !-!< LINE-ITEM : contains
+```
+```mermaid
+erDiagram
+    CUSTOMER !-?< ORDER : places
+    ORDER !-!< LINE-ITEM : contains
+```
+
+## Syntax
+
+### Entities and Relationships
+
+To be completed