tcg/README: document tcg_gen_goto_tb restrictions See http://lists.nongnu.org/archive/html/qemu-devel/2012-09/msg03196.html for the whole story. Signed-off-by: Max Filippov <jcmvbkbc@gmail.com> Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>