Add/block icon component readme

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

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+# Block Icon
+
+The BlockIcon component provides an icon for blocks in different places in the editor: the toolbar of a selected block, the placeholders of certain blocks (gallery, image), the block insertion panel of the editor or the left sidebar.
+
+The rendered an [Icon](https://github.com/WordPress/gutenberg/tree/master/packages/components/src/icon) component with default styles.
+
+![Image block icons in various places](https://make.wordpress.org/core/files/2020/08/image-block-icons-in-various-places.png)
+
+## Table of contents
+
+1. [Development guidelines](#development-guidelines)
+2. [Related components](#related-components)
+
+
+## Development guidelines
+
+### Usage
+
+Renders a block icon with default style.
+
+```jsx
+import { BlockIcon } from '@wordpress/block-editor';
+
+const MyBlockIcon = () => <BlockIcon icon={ icon } />
+```
+
+## 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.