FAQ: Mention packet filter incompatibility

Co-authored-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Pritesh Kothari <pritesh.kothari@cisco.com>
Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>