Quick Start Guide: Refer to Wiki for assistance

Updated the document's "Abstract" to refer to the Midonet Wiki's "Mailing lists
and IRC" page (instead of only mentioning IRC and linking to freenode).

Change-Id: Ie89b6e4e604adafd0dd2f8e7b3f8ea94c7d2d07f
Signed-off-by: Jan Hilberath <jan@midokura.com>