debian: Better document how to get a kernel module in README.Debian.

This moves existing text from INSTALL.Debian into the README.Debian
installed as part of the openvswitch-switch package, and adds a reference
from the former to the latter.

Reported-by: Brian Candler <b.candler@pobox.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Gurucharan Shetty <gshetty@nicira.com>