Add readme for SkipToSelectedBlock component

packages/block-editor/src/components/skip-to-selected-block/README.md

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+# SkipToSelectedBlock
+
+`SkipToSelectedBlock` provides a way for keyboard and assistive technologies users to jump back to the currently selected block.
+
+The component renders as a visually hidden, secondary button with the text, 'Skip to the selected block'. When clicked it will set [focus](https://developer.mozilla.org/en-US/docs/Web/API/HTMLOrForeignElement/focus) on the currently selected block. This means that the currently selected block will now receive all keyboard and similar events by default until focus is moved elsewhere.
+
+If there are multiple blocks selected, the focus will be set on the block that was selected first.
+
+## Usage
+
+Render a `<SkipToSelectedBlock />` component passing in the currently selected block's `clientId`.
+
+```jsx
+function render( { clientId } ) {
+	return (
+		<div>
+			<SkipToSelectedBlock selectedBlockClientId={ clientId } />
+		</div>
+	);
+}
+```
+
+_Note:_
+
+## Props
+
+### `selectedBlockClientId`
+
+-   **Type:** `String`
+-   **Required** `true`
+-   **Default:** `undefined`
+
+The `selectedBlockClientId` is passed to `useBlockRef` inside the component to correctly reference the currently selected block.
+
+If a `clientId` is not provided, the component will return `null`.
+
+## Examples
+
+This component is currently used in the [block inspector](https://github.com/WordPress/gutenberg/tree/trunk/packages/block-editor/src/components/block-inspector) in the sidebar, at the bottom right corner of the screen.