[PATCH] update to the faq regarding the "touch: not found" error during buildworld

[Niclas Zeising]

Hi!
This is an attempt to update the faq with an entry regarding the "touch: 
not found" error. There is seen from time to time people asking and 
wondering about this strange and non-informative error, so I think an 
addition to the faq is good.

Feel free to change anything you like in the diff!
Regards!
//Niclas