Panel: Add design documentation to readme

packages/components/src/panel/README.md

@@ -1,71 +1,144 @@
 # Panel
+ Panels expand and collapse multiple sections of content.
+
+![](https://make.wordpress.org/design/files/2019/03/panel.png)
+
+## Table of contents
+
+1. [Design guidelines](#design-guidelines)
+2. [Development guidelines](#development-guidelines)
+3. [Related components](#related-components)
+
+## Design guidelines
+
+### Anatomy
+
+A Panel is a single section of content that can be expanded or collapsed as needed.
+
+![Panel anatomy](https://make.wordpress.org/design/files/2019/03/panel-anatomy.png)
+
+1. Panel
+2. Body
+3. Divider
+
+### Usage
+
+Panels show and hide details of list items by expanding and collapsing list content vertically. Panels help users see only the content they need.
+
+#### When to use Panels
+
+Use Panels when it’s helpful to:
+
+- See an overview of multiple, related sections of content.
+- Show and hide those sections as needed.
+- Hide information that is lower priority that users don’t need to see all the time.
+- View more than one section at a time.
+
+Consider an alternative component when:
+
+- There’s crucial information or error messages that require immediate action.
+- You need to quickly switch between only a few sections (consider using Tabs instead).
+
+### Behavior
+
+#### Expand and collapse
+
+Show and hide details of existing panel items by expanding and collapsing list content vertically.
+
+![](https://make.wordpress.org/design/files/2019/03/panel-expand.gif)
+
+Collapsible panels are indicated with a caret icon that is flipped when expanded. Carets are preferable over a plus or arrow icon, because a plus indicates addition, and arrows are closely related to navigation.
+
+Panels should be expanded by default if the content is important or essential. Panels that are open by default should appear at the top.
+
+## Development guidelines
 
 The `Panel` creates a container with a header that can take collapsible `PanelBody` components to easily create a user friendly interface for affecting state and attributes.
 
-## Sub-Components
+### Usage
 
-### Panel
+```jsx
+import { Panel, PanelBody, PanelRow } from '@wordpress/components';
+
+const MyPanel = () => (
+	<Panel header="My Panel">
+		<PanelBody
+			title="My Block Settings"
+			icon="welcome-widgets-menus"
+			initialOpen={ true }
+		>
+			<PanelRow>
+				My Panel Inputs and Labels
+			</PanelRow>
+		</PanelBody>
+	</Panel>
+);
+```
+
+### Sub-Components
 
-#### Props
+#### Panel
 
-##### className
+##### Props
+
+###### className
 
 The class that will be added with `components-panel`. If no `className` is passed only `components-panel__body` and `is-opened` is used.
 
 - Type: `String`
 - Required: No
 
-##### header
+###### header
 
 Title of the `Panel`. Text will be rendered inside an `<h2>` tag.
 
 - Type: `String`
 - Required: No
 
 ---
-### PanelBody
+#### PanelBody
 
 The `PanelBody` creates a collapsible container that can be toggled open or closed. 
 
-#### Props
+##### Props
 
-##### title
+###### title
 
 Title of the `PanelBody`. This shows even when it is closed.
 
 - Type: `String`
 - Required: No
 
 
-##### opened
+###### opened
 
 If opened is true then the `Panel` will remain open regardless of the `initialOpen` prop and the panel will be prevented from being closed.
 
 - Type: `Boolean`
 - Required: No
 
-##### className
+###### className
 
 The class that will be added with `components-panel__body`, if the panel is currently open, the `is-opened` class will also be passed to the classes of the wrapper div. If no `className` is passed then only `components-panel__body` and `is-opened` is used.
 
 - Type: `String`
 - Required: No
 
-##### icon
+###### icon
 
 An icon to be shown next to the `PanelBody` title.
 
 - Type: `String`
 - Required: No
 
-##### onToggle
+###### onToggle
 
 A function that is called when the user clicks on the `PanelBody` title after the open state is changed.
 
 - Type: `function`
 - Required: No
 
-##### initialOpen
+###### initialOpen
 
 Whether or not the panel will start open.
 
@@ -74,49 +147,32 @@ Whether or not the panel will start open.
 - Default: true
 
 ---
-### PanelRow
+#### PanelRow
 
 The is a generic container for panel content. Default styles add a top margin and arrange items in a flex row.
 
-#### Props
+##### Props
 
-##### className
+###### className
 
 The class that will be added with `components-panel__row`.  to the classes of the wrapper div. If no `className` is passed only `components-panel__row` is used.
 
 - Type: `String`
 - Required: No
 ---
 
-### PanelHeader
+#### PanelHeader
 
 This is a simple container for a header component. This is used by the `Panel` component under the hood, so it does not typically need to be used.
 
-#### Props
+##### Props
 
-##### label
+###### label
 
 The text that will be rendered as the title of the `Panel`. Will be rendered in an `<h2>` tag.
 
 - Type: `String`
 - Required: No
 
-## Usage
-
-```jsx
-import { Panel, PanelBody, PanelRow } from '@wordpress/components';
-
-const MyPanel = () => (
-	<Panel header="My Panel">
-		<PanelBody
-			title="My Block Settings"
-			icon="welcome-widgets-menus"
-			initialOpen={ true }
-		>
-			<PanelRow>
-				My Panel Inputs and Labels
-			</PanelRow>
-		</PanelBody>
-	</Panel>
-);
-```
+## Related components
+- To divide related sections of content accessed by a horizontal menu, use  `TabPanel`