Add info + link to known dependencies plugin

@see https://github.com/x-team/wp-plugin-dependencies/issues/34
That way it will show up in a block with a different background color (like the code samples), making it clear it's additional info, not info about the main plugin.

See https://wordpress.org/plugins/known-plugin-dependencies/ for an example - the block which says 'You'll need to have version 1.3 or higher of the WordPress Plugin Dependencies plugin installed for this plugin to have any effect.' is done with the `>`