NEW: Adding more user documentation with a FAQ

There's only mention of one benefit here, should probably say "One of the benefits..." if there's only one, or add some more if you can think of other reasons.

Ideally we would also describe the limitations of subsites as much as it's benefits, specifically that it's *not* a way of running multiple sites on the same machine, i.e. different vhosts on the same box. Nailing down some good scenarios that a user would understand would help greatly here, e.g. a scenario on subsites working well, and another where it doesn't work too well (e.g. loads of specific page types that don't share between main and other subsites, perhaps?)

The other limitation I see is if you have main site CMS permission, you effectively have access to all subsites. AFAICT, that is the case, right? This may not be apparent to someone who's evaluating the use of subsites, but we should document that limitation.

I think we should also outline the scenario where a subsite needs specific page types, and how you would hide those so they don't show up other subsites ("Disallow page types?" field when editing a Subsite record.)
Note that there's currently no way to hide subsite-specific page types on the main site, that is currently a limitation, but I could see that being useful if you've got subsite page types that shouldn't be used on the main site.