Add section in documentation on email-safe CSS

[jsodeman]

CSS styling is used in many places in the documentation, but there isn't any content warning that not all CSS is supported in email clients.

I think a short section would be useful both as a warning and as a resource. It could reference URLs like:

• https://www.campaignmonitor.com/css/
• https://designmodo.com/html-css-emails/
• https://www.caniemail.com/

[iRyusa]

Yep you're right we should maybe point to Can I Email as it's the most up to date resource we have