doc(er): add documentation on multiple key constraints

demos/er.html

@@ -85,6 +85,30 @@
     >
     <hr />
 
+    <pre class="mermaid">
+      erDiagram
+        CAR ||--o{ NAMED-DRIVER : allows
+        CAR {
+          string registrationNumber PK
+          string make
+          string model
+          string[] parts
+        }
+        PERSON ||--o{ NAMED-DRIVER : is
+        PERSON {
+          string driversLicense PK "The license #"
+          string(99) firstName "Only 99 characters are allowed"
+          string lastName
+          string phone UK
+          int age
+        }
+        NAMED-DRIVER {
+          string carRegistrationNumber PK,FK
+          string driverLicence PK,FK
+        }
+        MANUFACTURER only one to zero or more CAR : makes
+    </pre>
+
     <script src="./mermaid.js"></script>
     <script type="module">
       mermaid.initialize({