Add documentation on alternative way of installing linkerd on dc/os

[dadjeibaah]

The current linkerd-dcos.json install file used in linkerd examples can prevent the admin shutdown API feature in Linkerd from working. This documentation adds a note that describes a new way to set up a DC\OS linkerd configuration so admin shutdown API feature works in a DC\OS environment.

Signed-off-by: Dennis Adjei-Baah dennis@buoyant.io