Reachability explanation

A common thought process problem I see is in Issues is relying on Reachability's initial state reporting. I wince every time I see this, since it's not supposed to matter. Under almost every circumstance, you're supposed to **try** first and check after.

I wrote up a short explanation of this. You may want to rewrite this, but I think something *like* this should be included to reduce people trying to hang themselves on a useful class by using it inappropriately.