Update docs to mention thumbnail plugin requirements.
Update docs to mention thumbnail plugin requirements.

whilesoftware/haxeflixel.com,gamedevsam/haxeflixel.com,IBwWG/haxeflixel.com,whilesoftware/haxeflixel.com,gamedevsam/haxeflixel.com,joackobengochea/haxeflixel.com,IBwWG/haxeflixel.com,IBwWG/haxeflixel.com,whilesoftware/haxeflixel.com,joackobengochea/haxeflixel.com,gamedevsam/haxeflixel.com,joackobengochea/haxeflixel.com