add docs for shadow dom options

Author: janbiasi

README.md

@@ -5,14 +5,12 @@ Previous versions (< 3.0.0) implemented the v0 proposal, which was only implemen
 
 ## Usage
 
-Import `register` and call it with your component, a tag name<strong>*</strong>, and a list of attribute names you want to observe:
+Import `register` and call it with your component, a tag name<strong>\*</strong>, and a list of attribute names you want to observe:
 
 ```javascript
 import register from 'preact-custom-element';
 
-const Greeting = ({ name = 'World' }) => (
-  <p>Hello, {name}!</p>
-);
+const Greeting = ({ name = 'World' }) => <p>Hello, {name}!</p>;
 
 register(Greeting, 'x-greeting', ['name']);
 ```
@@ -42,15 +40,15 @@ import register from 'preact-custom-element';
 
 // <x-greeting name="Bo"></x-greeting>
 class Greeting extends Component {
-  // Register as <x-greeting>:
-  static tagName = 'x-greeting';
+	// Register as <x-greeting>:
+	static tagName = 'x-greeting';
 
-  // Track these attributes:
-  static observedAttributes = ['name'];
+	// Track these attributes:
+	static observedAttributes = ['name'];
 
-  render({ name }) {
-    return <p>Hello, {name}!</p>;
-  }
+	render({ name }) {
+		return <p>Hello, {name}!</p>;
+	}
 }
 register(Greeting);
 ```
@@ -60,15 +58,41 @@ If no `observedAttributes` are specified, they will be inferred from the keys of
 ```js
 // Other option: use PropTypes:
 function FullName(props) {
-  return <span>{props.first} {props.last}</span>
+	return (
+		<span>
+			{props.first} {props.last}
+		</span>
+	);
 }
 FullName.propTypes = {
-  first: Object,   // you can use PropTypes, or this
-  last: Object     // trick to define untyped props.
+	first: Object, // you can use PropTypes, or this
+	last: Object, // trick to define untyped props.
 };
 register(FullName, 'full-name');
 ```
 
+### Using the shadow DOM
+
+It is possible to attach a shadow root to the custom element to have your component rendered in a separate subtree.
+Customization of the shadow root `mode` is also possible on element basis.
+Read more about the shadow DOM on [MDN](https://developer.mozilla.org/en-US/docs/Web/API/ShadowRoot).
+
+```tsx
+function WithShadow(props) {
+	return <span>Hello from shadow root!</span>;
+}
+
+// use shadow root in "open" mode
+register(FullName, 'full-name', [], {
+	shadow: true,
+});
+
+// use shadow root in "closed" mode
+register(FullName, 'full-name-encapsulated', [], {
+	shadow: true,
+	mode: 'closed',
+});
+```
 
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