Update Toolbar component readme (WordPress#13316)

* UpdateToolbar component readme

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

Thanks @cburton4 and @alexislloyd for drafting this.

* Update heading levels

There should only be one H1: the page title. Adjusting the rest of the levels to match.

* Switch from bold text to headings.

packages/components/src/toolbar/README.md

@@ -1,6 +1,45 @@
 # Toolbar
 
-## Usage
+Toolbar can be used to group related options. To emphasize groups of related icon buttons, a toolbar should share a common container.
+
+![Toolbar component above an image block](https://wordpress.org/gutenberg/files/2019/01/s_96EC471FE9C9D91A996770229947AAB54A03351BDE98F444FD3C1BF0CED365EA_1541782974545_ButtonGroup.png)
+
+## Table of contents
+
+1. [Design guidelines](#design-guidelines)
+2. [Development guidelines](#development-guidelines)
+
+## Design guidelines
+
+### Usage
+
+#### Selected action
+
+Only one option in a toolbar can be selected and active at a time. Selecting one option deselects any other.
+
+### Best practices
+
+Toolbars should:
+
+- **Clearly communicate that clicking or tapping will trigger an action.**
+- **Use established colors appropriately.** For example, only use red for actions that are difficult or impossible to undo.
+- When used with a block, have a consistent location above the block. **Otherwise, have a consistent location in the interface.**
+
+![A toolbar attached to the top left side of a block. (1. Toolbar, 2. Block)](https://wordpress.org/gutenberg/files/2019/01/s_D8D19E5A314C2D056B8CCC92B2DB5E27164936A0C5ED98A4C2DFDA650BE2A771_1542388042335_toolbar-block.png)
+
+### States
+
+#### Active and available toolbars
+A toolbar’s state makes it clear which icon button is active. Hover and focus states express the available selection options for icon buttons in a toolbar.
+
+![Toolbar component](https://wordpress.org/gutenberg/files/2019/01/s_96EC471FE9C9D91A996770229947AAB54A03351BDE98F444FD3C1BF0CED365EA_1541784539545_ButtonGroup.png)
+
+#### Disabled toolbars
+Toolbars that cannot be selected can either be given a disabled state, or be hidden.
+
+## Development guidelines
+
+### Usage
 
 ```jsx
 import { Toolbar } from '@wordpress/components';