INSTALL: Describe how to use a separate build directory.

CC: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
CC: Andy Zhou <azhou@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>
Acked-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>