Document Debian on QNAP TS-12x, TS-22x and TS-42x

Thanks to Bernd Büchs for testing Debian on a TS-221.
USB 3.0 on QNAP is supported in Debian 7.1
Don't refer to wheezy as Debian 7.0 but as Debian 7
Add a note about unknown flash chip on QNAP devices