Add documentation for dsr option for Overlay Networks

[arkodg]

moby/libnetwork#2270 adds support to create a overlay network
with DSR (direct server return) mode which does an L2 forward and skips a SNAT rewrite keeping the L3 headers intact

Usage docker network create -d overlay --opt dsr dsr-net

File: network/network-tutorial-overlay.md

[arkodg]

cc: @thaJeztah

[docker-robott]

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[thaJeztah]

/lifecycle frozen