Add pip support info to readme

Closes cookpete/react-player#694

README.md

@@ -75,7 +75,7 @@ Prop | Description | Default
 `style` | Add [inline styles](https://facebook.github.io/react/tips/inline-styles.html) to the root element | `{}`
 `progressInterval` | The time between `onProgress` callbacks, in milliseconds | `1000`
 `playsinline` | Applies the `playsinline` attribute where supported | `false`
-`pip` | Set to `true` or `false` to enable or disable [picture-in-picture mode](https://developers.google.com/web/updates/2018/10/watch-video-using-picture-in-picture) | `false`
+`pip` | Set to `true` or `false` to enable or disable [picture-in-picture mode](https://developers.google.com/web/updates/2018/10/watch-video-using-picture-in-picture)<br/>&nbsp; ◦ &nbsp;Only available when playing file URLs in [certain browsers](https://caniuse.com/#feat=picture-in-picture) | `false`
 `wrapper` | Element or component to use as the container element | `div`
 `config` | Override options for the various players, see [config prop](#config-prop)