update README with new build requirements (gcc 4.6, MSVC 11)
update README with new build requirements (gcc 4.6, MSVC 11)

lwahlmeier/avian,minor-jason/avian,bigfatbrowncat/avian-pack.avian,lostdj/avian,MaartenR/avian,dicej/avian,bigfatbrowncat/avian-pack.avian,lostdj/avian,lwahlmeier/avian,dicej/avian,ucdseniordesign/avian,minor-jason/avian,bgould/avian,marcinolawski/avian,MaartenR/avian,ucdseniordesign/avian,joshuawarner32/avian,marcinolawski/avian,bgould/avian,bigfatbrowncat/avian-pack.avian,joshuawarner32/avian,bgould/avian,lostdj/avian,marcinolawski/avian,MaartenR/avian,minor-jason/avian,ucdseniordesign/avian,marcinolawski/avian,bigfatbrowncat/avian-pack.avian,lostdj/avian,dicej/avian,lwahlmeier/avian,joshuawarner32/avian,MaartenR/avian,minor-jason/avian,joshuawarner32/avian,dicej/avian,ucdseniordesign/avian,bgould/avian,lwahlmeier/avian