!SECTION Launching an ArangoDB cluster using Docker containers

Essentially describe that the standard image can be used and explain
needed command line options. Details can be found in 
`scripts/startLocalDockerCluster.sh`.

Explain which command line options play which role in connecting
the cluster.

Link to script in github repo.

Explain that later more servers can be added by launching more docker 
containers.

Reiterate that this allows for synchronous replication with automatic
failover but that it cannot replace failed instances or automatically
scale up or down.