[video_player] Update README to mention web support.
[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.

flutter/plugins,tvolkert/plugins,mehmetf/plugins,flutter/plugins,mehmetf/plugins,flutter/plugins,flutter/plugins,flutter/plugins,flutter/plugins,tvolkert/plugins,mehmetf/plugins,flutter/plugins,mehmetf/plugins,tvolkert/plugins,flutter/plugins,tvolkert/plugins,mehmetf/plugins