Add note to make it clear that premailer-rails3 pulls in the email.css without any help

Twice in the last year I've started using premailer and both times I 
had a `<%= stylesheet_link_tag('email') %>` in my template, thinking 
I needed it.  The result is that sending email takes minutes instead 
of being near-instant and made me want to back out my use of premailer.

I'm hoping this doc change will spare me and others that same headache in the future.