Adds instance ID

app/docs/md/elements/state/instance-id.md

@@ -2,4 +2,18 @@
 title: Instance ID
 ---
 
+`instanceID` is a unique identifier that is generated per Custom Element instance. This enables you to differentiate between multiple instances of the same element on a page. It can also be useful when using a  string based diffing library like [Morphdom](https://github.com/patrick-steele-idem/morphdom) that require unique identifiers for DOM diffing.
 
+```javascript
+export default function MyCard({ html, state }) {
+  const { attrs={}, instanceID='' } = state
+  const { content='', heading='' } = attrs
+
+  return html`
+<figure id="figure-${instanceID}">
+  <h2>${heading}</h2>
+  <p>${content}</p>
+</figure>
+  `
+}
+```