Add note to the install doc about bridge MAC issues

Thanks to Faidon Liambotis for explaining this on the external IRC
channel.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Faidon Liambotis <paravoid@gmail.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>