feat(docs): documented as prop implementation

Author: codebender828

README.md

@@ -171,7 +171,7 @@ Remember to register `ThemeProvider` locally.
 
 Add your `ThemeProvider` component:
 
-```JSX
+```vue
   <theme-provider :theme="{
     primary: 'palevioletred'
   }">
@@ -240,6 +240,26 @@ const Button = styled.button`
 const Link = Button.withComponent('a')
 ```
 
+### Polymorphic `as` prop
+If you want to keep all the styling you've applied to a component but just switch out what's being ultimately rendered (be it a different HTML tag or a different custom component), you can use the "as" prop to do this at runtime. Another powerful feature of the `as` prop is that it preserves styles if the lowest-wrapped component is a `StyledComponent`.
+
+**Example**
+In `Component.js`
+```js
+//  Renders a div element by default.
+const Component = styled('div', {})``
+```
+Using the `as` prop in another template/component would be as shown below.
+```vue
+<template>
+  <!-- Component will render a button instead of a div -->
+  <Component as="button" color="red">
+    Button
+  </Component>
+</template>
+```
+This sort of thing is very useful in use cases like a navigation bar where some of the items should be links and some just buttons, but all be styled the same way.
+
 ### injectGlobal
 
 A helper method to write global CSS. Does not return a component, adds the styles to the stylesheet directly.