added readme to matrix alignment toolbar

packages/block-editor/src/components/block-alignment-matrix-toolbar/README.md

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+# Alignment Matrix Toolbar
+The alignment matrix allows users the ability to change text alignment quickly and intuitively.
+
+![Button components](https://i.imgur.com/PxYkgL5.png)
+
+## Table of contents
+
+- [Alignment Matrix Toolbar](#alignment-matrix-toolbar)
+  - [Table of contents](#table-of-contents)
+  - [Design guidelines](#design-guidelines)
+    - [Usage](#usage)
+    - [Best practices](#best-practices)
+  - [Development guidelines](#development-guidelines)
+    - [Usage](#usage-1)
+    - [Props](#props)
+
+## Design guidelines
+
+### Usage
+
+The alignment matrix is a specialized tool, and it's used in the cover block.
+
+![Cover](https://i.imgur.com/nJjqen8.png)
+
+As an example, here's the matrix alignment tool in action.
+
+![center](https://i.imgur.com/0Ce1fZm.png)
+
+
+![rop_right](https://i.imgur.com/yGGf6IP.png)
+
+### Best practices
+
+The matrix alignment tool should:
+
+- **Folow graphic design alignment guidelines** ; text alignment is important in graphic design. You can find tutorials online about alignment principles in graphic design, but [here](https://www.printwand.com/blog/basic-alignment-principles-in-graphic-design-with-examples) is an example.
+
+
+## Development guidelines
+
+### Usage
+
+```jsx
+// This is a paraphrased example from the cover block
+import { 
+    BlockControls,
+    __experimentalBlockAlignmentMatrixToolbar as BlockAlignmentMatrixToolbar
+} from "@wordpress/block-editor";
+
+const controls = (
+  <>
+    <BlockControls>
+      <BlockAlignmentMatrixToolbar
+        label={ __( 'Change content position' ) }
+        value={ contentPosition }
+        onChange={ ( nextPosition ) =>
+          setAttributes( { contentPosition: nextPosition } )
+        }
+              />
+          </BlockControls>
+      </>
+}
+```
+
+### Props
+
+
+Name | Type | Default | Description
+--- | --- | --- | ---
+`label` | `string` | `Change matrix alignment` | concise description of tool's functionality.
+`onChange` | `function` | `noop` | the function to execute upon a user's change of the matrix state
+`value` | `string` | `center` | describes the content alignment location and can be `top`, `right`, `bottom`, `left`, `topRight`, `bottomRight`, `bottomLeft`, `topLeft`
+