doc: add Windows processor group behaviour and Windows target option

doc: add Windows processor group behaviour and Windows target option

- Add information on targeting the build against different versions of
  Windows to the README
- Stop listing -i686 packages in the 64 bit section of the README and
  add a note to about them to the 32 bit build section
- Rejig cpus_allowed to be before cpumask in the documentation to make
  people more likely to read that section first
- Make it clear that CPUs specified in the cpus_allowed section are
  indexed from 0 (and not 1)
- Add information about how processor group awareness when setting CPUs
  is only available when using a build targeted at Windows 7 (because
  only then do we have the processor group APIs available to us) and how
  selecting a CPU sets the processor group too (because we concatanate
  the CPUs from all the processor groups together)

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>