add erd diagram

Author: zachauten

docs/ERD.md

@@ -0,0 +1,10 @@
+# Example ERD diagram using (mermaid)[https://github.blog/developer-skills/github/include-diagrams-markdown-files-mermaid/]
+```mermaid
+	---
+	title: Order example
+	---
+	erDiagram
+	    CUSTOMER ||--o{ ORDER : places
+	    ORDER ||--|{ LINE-ITEM : contains
+	    CUSTOMER }|..|{ DELIVERY-ADDRESS : uses
+```