[video_player] Update README to mention web support.

This change updates the core README to call out web is also supported for this plugin.

Explicitly tells users not to use the constructor when using the plugin for web.
diff --git a/packages/video_player/video_player/README.md b/packages/video_player/video_player/README.md
index 1b9eb80..88b1c0a 100644
--- a/packages/video_player/video_player/README.md
+++ b/packages/video_player/video_player/README.md
@@ -2,7 +2,7 @@
 
 [![pub package](https://img.shields.io/pub/v/video_player.svg)](https://pub.dartlang.org/packages/video_player)
 
-A Flutter plugin for iOS and Android for playing back video on a Widget surface.
+A Flutter plugin for iOS, Android and Web for playing back video on a Widget surface.
 
 ![The example app running in iOS](https://github.com/flutter/plugins/blob/master/packages/video_player/video_player/doc/demo_ipod.gif?raw=true)
 
@@ -44,7 +44,7 @@
 
 This plugin compiles for the web platform since version `0.10.5`, in recent enough versions of Flutter (`>=1.12.13+hotfix.4`).
 
-> The Web platform does **not** suppport `dart:io`, so attempts to create a `VideoPlayerController.file` will throw an `UnimplementedError`.
+> The Web platform does **not** suppport `dart:io`, so avoid using the `VideoPlayerController.file` constructor for the plugin. Using the constructor attempts to create a `VideoPlayerController.file` that will throw an `UnimplementedError`.
 
 Different web browsers may have different video-playback capabilities (supported formats, autoplay...). Check [package:video_player_web](https://pub.dev/packages/video_player_web) for more web-specific information.