Cilium: Add Cilium blurb to Mesos cni.md docs

[danwent]

Now that there is a getting started guide for Mesos + Cilium, we should update the Mesos upstream docs to point to Cilium as an option: https://github.com/apache/mesos/blob/master/docs/cni.md

[techcet]

Draft: https://github.com/techcet/mesos/blob/cilium-mesos/docs/cni.md#a-cilium-network

[tgraf]

@techcet Looks great.

[tgraf]

We might want to link to actual installation instructions for Mesos. This is currently hidden in the Vagrantfile and the start_marathon.sh in examples/mesos. We should add a chapter to the Installation Guide that is Mesos specific. Without knowing the exact instructions, we may actually be able to include a short version of the instructions directly into the Mesos docs.

[techcet]

Thanks for the feedback. I missed it before creating the PR and it got merged pretty quickly. It might take a bit longer to do your suggestion, so in the mean time, we're live on apache/mesos:
https://github.com/apache/mesos/commit/07c9c27aaf13d5eb0dfd195be7e3449de09003bd

First commit to external org!