Update documentation

[MichalKrakow]

Just after digging into AttachmentEmbedder.php i found out that files must be served from public folder...

$filePath = str_replace(url('/'), public_path('/'), $url);

this would be like the first thing to include in the documentation.

[eduardokum]

This is if using url, if using the url is obviously that need be public.

I'll be warning this in the documentation to be clearer

Thanks