BUILD.Windows: Add a TODO section.

And mention the lack of atomics support on Windows as the
first item.

Suggested-by: Jarno Rajahalme <jrajahalme@nicira.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Acked-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>