Add readme for the line height control component

packages/block-editor/src/components/line-height-control/README.md

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+# Line Height Control
+
+The `LineHeightControl` component adds a lineHeight attribute to the core Paragraph and Heading blocks. This allows to update a paragraph line height directly from the block editor interface.
+
+![Line height control in a paragraph block](https://make.wordpress.org/core/files/2020/09/line-height-for-paragraph-block.png)
+
+_Note:_ It is worth noting that the line height setting option is an opt-in feature. [Themes need to declare support for it](https://developer.wordpress.org/block-editor/developers/themes/theme-support/#supporting-custom-line-heights) before it'll be available.
+
+## Table of contents
+
+1. [Development guidelines](#development-guidelines)
+2. [Related components](#related-components)
+
+## Development guidelines
+
+### Usage
+
+Renders the markup for the line height setting option in the block inspector.
+
+```jsx
+import { KeyboardShortcuts } from '@wordpress/block-editor';
+const MyLineHeightControl = () => (
+	<LineHeightControl value={ lineHeight } onChange={ onChange } />
+);
+```
+
+### Props
+
+#### `value`
+
+-   **Type:** `String` or `Number` or `Undefined`
+
+The value of the line height.
+
+#### `onChange`
+
+-   **Type:** `Function`
+
+A callback function that handles the application of the line height value.
+
+
+## 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.