Include docs for deployments with Fog gem

Have been struggling with Amazon S3 configuration for Nanoc deploy so this is the brain dump for others.
I find “any Fog provider” a bit confusing. What about this:

> … any cloud hosting service supported by &lt;span class="productname">fog&lt;/span> …

I’d also turn the first occurrence of _fog_ into a link to [fog.io](http://fog.io).