Add README.md for PublishDateTimePicker

packages/block-editor/src/components/publish-date-time-picker/README.md

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+# `PublishDateTimePicker`
+
+`<PublishDateTimePicker />` is a component used to select the date and time that
+a post will be published. It wraps the `<DateTimePicker />` component found in
+`@wordpress/components` and adds additional post-specific controls.
+
+See [the documentation for
+DateTimePicker](https://github.com/WordPress/gutenberg/tree/trunk/packages/components/src/date-time)
+for more information.
+
+## Usage
+
+```jsx
+import { Dropdown, Button } from '@wordpress/components';
+import { __experimentalPublishDateTimePicker as PublishDateTimePicker } from '@wordpress/block-editor';
+import { useState } from '@wordpress/element';
+
+const MyDateTimePicker = () => {
+	const [ date, setDate ] = useState( new Date() );
+
+	return (
+		<Dropdown
+			renderToggle={ ( { isOpen, onToggle } ) => (
+				<Button
+					onClick={ onToggle }
+					aria-expanded={ isOpen }
+				>
+					Select post date
+				</Button>
+			) }
+			renderContent={ ( { onClose } ) => (
+				<PublishDateTimePicker
+					currentDate={ date }
+					onChange={ ( newDate ) => setDate( newDate ) }
+					onClose={ onClose }
+				/>
+			) }
+		/>
+	);
+};
+```
+
+## Props
+
+`PublishDateTimePicker` supports all of the props that
+[`DateTimePicker`](https://github.com/WordPress/gutenberg/tree/trunk/packages/components/src/date-time#Props)
+supports, plus:
+
+### onClose
+
+Called when the user presses the close button.
+
+- Type: `Function`
+- Required: Yes