Add BlockMover component readme (#24851)

packages/block-editor/src/components/block-mover/README.md

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+# Block mover
+
+Block movers allow moving blocks inside the editor using up and down buttons.
+
+![Block mover screenshot](https://make.wordpress.org/core/files/2020/08/block-mover-screenshot.png)
+
+
+## Table of contents
+
+1. [Development guidelines](#development-guidelines)
+2. [Related components](#related-components)
+
+
+## Development guidelines
+
+### Usage
+
+Shows the block mover buttons in the block toolbar.
+
+```jsx
+import { BlockMover } from '@wordpress/block-editor';
+const MyMover = () => <BlockMover clientIds={ [ clientId ] } />;
+```
+
+### Props
+
+#### clientIds
+
+Blocks IDs
+
+-   Type: `Array`
+
+
+## Related components
+Block Editor components are components that can be used to compose the UI of your block editor. Thus, they can only be used under a [BlockEditorProvider](https://github.com/WordPress/gutenberg/blob/master/packages/block-editor/src/components/provider/README.md) in the components tree.