Add readme for the multi selection component

packages/block-editor/src/components/multi-selection-inspector/README.md

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+# Multi Selection Inspector
+
+The `MultiSelectionInspector` component adds a card in the panel displayed by the [`BlockInspector`](https://github.com/WordPress/gutenberg/tree/master/packages/block-editor/src/components/block-inspector) component when several blocks are selected in the editor.
+
+This card contains information on the number of blocks selected, and in the case where one or more of these blocks contains text, the number of words they contain.
+
+![Multi selection inspector card](https://make.wordpress.org/core/files/2020/09/multi-selection-inspector-card.png)
+
+## Table of contents
+
+1. [Development guidelines](#development-guidelines)
+2. [Related components](#related-components)
+
+## Development guidelines
+
+### Usage
+
+Renders the multi selection inspector card.
+
+```jsx
+import { <MultiSelectionInspector /> } from '@wordpress/block-editor';
+
+const SelectedBlockCount = getSelectedBlockCount();
+
+if ( SelectedBlockCount > 1 ) {
+    const MyMultiSelectionInspector = () => <MultiSelectionInspector />;
+}
+```
+
+ _Note:_ In this example, we detect if more than one block is selected with `getSelectedBlockCount()` before using the `MultiSelectionInspector` component.
+
+## 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.