added README for the "caption" component (#52033)

* added README for the caption component

* Minor adjustments

* removed unnecesary space

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

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+## Caption
+
+The `Caption` component renders the [caption part](https://wordpress.org/documentation/article/gallery-block/#caption) of some blocks (image, gallery...).
+
+This component encapsulates the "caption" behaviour and styles over a `<RichText>` so it can be used in other components such as the `BlockCaption` component.
+
+## Table of contents
+
+1. [Development guidelines](#development-guidelines)
+2. [Related components](#related-components)
+
+## Development guidelines
+
+### Usage
+
+Renders a Caption area:
+
+```jsx
+import { Caption } from '@wordpress/block-editor';
+const BlockCaption = ( {
+	onBlur,
+	onChange,
+	onFocus,
+	isSelected,
+	shouldDisplay,
+	text,
+	insertBlocksAfter,
+} ) => (
+	<View >
+		<Caption
+			isSelected={ isSelected }
+			onBlur={ onBlur }
+			onChange={ onChange }
+			onFocus={ onFocus }
+			shouldDisplay={ shouldDisplay }
+			value={ text }
+			insertBlocksAfter={ insertBlocksAfter }
+		/>
+	</View>
+);
+```
+
+### Props
+
+The properties `isSelected`, `onBlur`, `onChange`, `onFocus`, `shouldDisplay`, `value`, `insertBlocksAfter` of this component are passed directly to their related props of its inner `<RichText>` component ([see detailed info about the RichText component's props](https://github.com/WordPress/gutenberg/blob/HEAD/packages/block-editor/src/components/rich-text/README.md)).
+
+## Related components
+
+Caption components is mostly used by the [`BlockCaption`](https://github.com/WordPress/gutenberg/blob/HEAD/packages/block-editor/src/components/block-caption) component.