Added information on how to find the partition number for autoboot.txt

Following the questions at https://github.com/raspberrypi/noobs/issues/150
Merge pull request #191 from RichardMorris/master

Added information on how to find the partition number for autoboot.txt
Minor miscellaneous changes for NOOBS v1.3.8
Fixes #204
NOOBS v1.3.9
Do not activate network on silentinstall

Closes #120 1.3 crashes with option "silentinstall"

(reason it crashes is actually that we are calling _qpd->hide()
when downloading the list of the Internet is complete, while _qpd
is not created on a silentinstall.
But no need for the network on a silent install anyway)