Remove mtools-host as it's unused and can cause build failures
Fixes #169
Merge pull request #170 from lurch/remove_mtools-host

Remove mtools-host as it's unused and can cause build failures
Update README.md

Clarify that Custom OSes won't work if you're using NOOBS to run multiple OSes.
Changes required for RISC OS to boot with a new partition offset
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