docs: Adds README for BlockEditVisuallyButton component.

packages/block-editor/src/components/block-edit-visually-button/README.md

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+# Block Edit Visually Button
+
+`BlockEditVisuallyButton` is a component that provides a toolbar button to switch from HTML editing mode to visual editing mode in the WordPress Block Editor (Gutenberg).
+
+## Description
+
+This component renders a toolbar button that allows users to switch from HTML editing mode back to the visual editor. The button only appears when a single block is selected and is currently in HTML editing mode.
+
+## Installation
+
+This component is available as part of the `@wordpress/block-editor` package and can be imported as:
+
+```js
+import { BlockEditVisuallyButton } from '@wordpress/block-editor';
+```
+
+## Properties
+
+### Required Props
+
+- `clientIds` (array): An array of block client IDs. The button will only be shown when exactly one block is selected (array length === 1).
+
+## Usage
+
+```jsx
+function MyBlockToolbar({ clientIds }) {
+  return (
+    <BlockControls>
+      <BlockEditVisuallyButton clientIds={clientIds} />
+    </BlockControls>
+  );
+}
+```
+
+## Behavior
+
+- The button is only rendered when:
+  - Exactly one block is selected
+  - The selected block is currently in HTML editing mode
+- Clicking the button toggles the block's editing mode from HTML to visual
+- The button is automatically hidden when in visual editing mode
+
+## Dependencies
+
+This component uses:
+
+- `@wordpress/components`: For `ToolbarButton` and `ToolbarGroup`
+- `@wordpress/i18n`: For internationalization
+- `@wordpress/data`: For state management
+- Block editor store
+
+## Related Components
+
+- `BlockModeToggle`: Complementary component that handles switching from visual to HTML mode
+- `BlockControls`: Parent component where this button is typically rendered
+
+## Notes
+
+- Uses the block editor data store to check and toggle block modes
+- Internationalization ready with default text "Edit visually"
+- Part of the block toolbar UI components suite