Update IconButton readme (#13063)

* Create icon-button readme

Adding documentation to describe the use and functionality of the IconButton component.

* Migrate changes over to the original icon-button readme file.

packages/components/src/icon-button/README.md

@@ -1,6 +1,38 @@
 # IconButton
 
-## Usage
+Icon buttons tell users what actions they can take and give them a way to interact with the interface. You’ll find them throughout a UI, particularly in places like:
+
+- Button groups
+- Toolbars
+
+![IconButton component](https://wordpress.org/gutenberg/files/2018/12/s_1D98B71431B26D39301DAD9ADD4189EA9DDF0B98AC93C77E7DE58172FFC06323_1541793686578_IconButton.png)
+
+![Toolbar component composed of attached IconButtons](https://wordpress.org/gutenberg/files/2018/12/s_1D98B71431B26D39301DAD9ADD4189EA9DDF0B98AC93C77E7DE58172FFC06323_1541793628237_Toolbar.png) 
+
+## Table of contents
+
+1. [Design guidelines](#design-guidelines)
+2. [Development guidelines](#development-guidelines)
+3. [Related components](#related-components)
+
+# Design guidelines
+
+## Best practices
+
+Icon buttons should:
+
+- **Choose a clear and accurate icon to represent the action.**
+- **Clearly communicate that clicking or tapping will trigger an action.**
+- **Use established colors appropriately.** For example, only use red buttons for actions that are difficult or impossible to undo.
+- **Have consistent locations in the interface.**
+
+## Anatomy
+
+Icon buttons have low emphasis. They don’t stand out much on the page, so they’re used for less-important actions. What’s less important can vary based on context, but it’s usually a supplementary action to the main action we want someone to take. Icon buttons are also useful when you don’t want to distract from the content.
+
+# Development guidelines
+
+##  Usage
 
 ```jsx
 import { IconButton } from '@wordpress/components';
@@ -12,3 +44,7 @@ const MyIconButton = () => (
 	/>
 );
 ```
+
+# Related components
+
+- The `Toolbar` component is composed of  `IconButton` components.