Add reference to K8sClusterManager in README

[omus]

As ClusterManagers.jl seems to be the first place people look for additional cluster managers it may be good to include references to external cluster managers as part of the table in the README.

Other managers that possible should be added could be:

• https://github.com/JuliaParallel/Elly.jl
• https://github.com/JuliaCloud/AWSClusterManagers.jl

There may be a better approach to improving discoverability of Julia cluster managers. This just seemed like an easy approach.

[omus]

I just remembered that ClusterManagers.jl is called out by the addprocs docstring which is probably why people look here for alternative cluster managers.