Documented the assumption that provisioning be idempotent

Here is an additional paragraph for the website docs, one that explains the vital assumption of idempotentcy in provisioning. Since I had never used any automated provisioning tools, I didn't understand that idempotentcy was expected, and so it took me a long to understand how vagrant could actually be used to share dev environments without having to completely rebuild the virtual machine whenever someone installed something new.

Hope this helps future newbs see the light faster. 

jack@flywheel.com