message: underlinenav system props docs
diff: # UnderlineNav
  +
  +To use UnderlineNav with [react-router](https://github.com/ReactTraining/react-router) or
  +[react-router-dom](https://www.npmjs.com/package/react-router-dom), pass
  +```is={NavLink}``` and omit the ```selected``` prop.
  +This ensures that the NavLink gets ```activeClassName='selected'```
  +
  +## Default example
  +
  ```.jsx
  <ExampleHeading>
  Using <Text fontFamily="mono">{'<UnderlineNavLink>'}</Text>
  </UnderlineNav>
  ```
  - To use UnderlineNav with [react-router](https://github.com/ReactTraining/react-router) or
  - [react-router-dom](https://www.npmjs.com/package/react-router-dom), pass
  - ```is={NavLink}``` and omit the ```selected``` prop.
  - This ensures that the NavLink gets ```activeClassName='selected'```
  +## System props
  +
  +UnderlineNav and UnderlineNavLink components get `COMMON` system props. Read our [System Props](/system-props) doc page for a full list of available props.
  export const meta = {displayName: 'UnderlineNav'}
  