Install guide, detail SSL requirements

[mglaman]

Let's add a complete SSL instructional page to the install docs:

In the past, you may have been instructed that you don’t need an SSL certificate if you’re using an off-site payment method. While this may in fact protect your customers’ payment card data, any time someone is accessing personally identifiable information or using a password to log in to a site, it should be protected by SSL. Without that, a malicious user of a public network can steal your customers’ data and use it to compromise their accounts anywhere else the same information or passwords are used.

The good news is that the last couple of years have seen a renaissance in security awareness and capability at the infrastructure level of the web. Not only can you get basic SSL protection for your customers via DNS hosting services like CloudFlare, but a variety of Drupal optimized hosting platforms like Platform.sh and Pantheon even bake free SSL certificates into their platforms for every customer. Accordingly, Drupal Commerce 2.x requires you to be using SSL to protect your site. Some payment gateway modules and integrated services may not even properly install without SSL due to their authentication requirements.

[rszrama]

Updated with the additional intro.