Update TextControl design documentation

packages/components/src/text-control/README.md

@@ -1,72 +1,118 @@
 # TextControl
+TextControl components let users enter and edit text.
 
-TextControl is normally used to generate text input fields. But can be used to generate other input types.
+![Unfilled and filled TextControl components](https://make.wordpress.org/design/files/2019/03/TextControl.png)
 
+## Table of contents
 
-## Usage
+1. [Design guidelines](#design-guidelines)
+2. [Development guidelines](#development-guidelines)
+3. [Related components](#related-components)
 
-Render a user interface to input the name of an additional css class.
+## Design guidelines
+
+### Usage
+
+#### When to use TextControls
+
+TextControls are best used for free text entry. If you have a set of predefined options you want users to select from, it’s best to use a more constrained component, such as a SelectControl, RadioControl, CheckboxControl, or RangeControl.
+
+Because TextControls are single-line fields, they are not suitable for collecting long responses. For those, use a text area instead.
+
+TextControls should:
+
+- Stand out and indicate that users can input information.
+- Have clearly differentiated states (selected/unselected, active/inactive).
+- Make it easy to understand the requested information and to address any errors.
+- Have visible labels; placeholder text is not an acceptable replacement for a label as it vanishes when users start typing.
+
+### Anatomy
+
+![Features of a TextControl component with numbered labels](https://make.wordpress.org/design/files/2019/03/TextControl-Anatomy.png)
+
+1. Label
+2. Input container
+3. Input text
+
+#### Label text
+Label text is used to inform users as to what information is requested for a text field. Every text field should have a label. Label text should be above the input field, and always visible. 
+
+#### Containers
+Containers improve the discoverability of text fields by creating contrast between the text field and surrounding content.
+
+**Do**
+
+![A TextControl with a stroke around the container to clearly indicate the input area](https://make.wordpress.org/design/files/2019/03/TextControl-Do.png)
+
+A stroke around the container clearly indicates that users can input information.
 
-```jsx
-import { TextControl } from '@wordpress/components';
-import { withState } from '@wordpress/compose';
+**Don’t** 
 
-const MyTextControl = withState( {
-	className: '',
-} )( ( { className, setState } ) => (
-	<TextControl
-		label="Additional CSS Class"
-		value={ className }
-		onChange={ ( className ) => setState( { className } ) }
-	/>
-) );
-```
+![A TextControl without a clear visual marker to indicate the input area](https://make.wordpress.org/design/files/2019/03/TextControl-Dont.png)
 
-## Props
+Don’t use unclear visual markers to indicate a text field.
+
+## Development guidelines
+
+### Usage
+
+Render a user interface to input the name of an additional css class.
+
+    import { TextControl } from '@wordpress/components';
+    import { withState } from '@wordpress/compose';
+
+    const MyTextControl = withState( {
+        className: '',
+    } )( ( { className, setState } ) => ( 
+        <TextControl
+            label="Additional CSS Class"
+            value={ className }
+            onChange={ ( className ) => setState( { className } ) }
+        />
+    ) );
+
+### Props
 
 The set of props accepted by the component will be specified below.
 Props not included in this set will be applied to the input element.
 
-### label
-
+#### label
 If this property is added, a label will be generated using label property as the content.
 
 - Type: `String`
 - Required: No
 
-### help
-
+#### help
 If this property is added, a help text will be generated using help property as the content.
 
-- Type: `String|WPElement`
+- Type: `String`
 - Required: No
 
-### type
-
+#### type
 Type of the input element to render. Defaults to "text".
 
 - Type: `String`
 - Required: No
 - Default: "text"
 
-### value
-
+#### value
 The current value of the input.
 
 - Type: `Number`
 - Required: Yes
 
-### className
-
+#### className
 The class that will be added with "components-base-control" to the classes of the wrapper div.
 If no className is passed only components-base-control is used.
 
 - Type: `String`
 - Required: No
 
-### onChange
-
+#### onChange
 A function that receives the value of the input.
 
 - Type: `function`
 - Required: Yes
+
+## Related components
+- To offer users more constrained options for input, use SelectControl, RadioControl, CheckboxControl, or RangeControl.