Add documentation for dpctl command used in Mininet; add ToC

dpctl tool is an utility that provides basic control over OpenFlow
switches that support passive listening port.
This commit also introduces Table of Contents to the document describing
Mininet/LINC integration.
(Motivated by FlowForwarding/LINC-Switch#250).
Merge pull request #284 from FlowForwarding/mininet_dpctl

Add documentation for dpctl command used in Mininet; add ToC