State minimal recommended specs for multi-server (#481, @Filius-Patris)

[Filius-Patris]

Problem:

See this issue: #315. In resource constrained environments, starting k3d with more than one servers fails.

Solution:

Document the minimal requirements. They are based of empirical research documented here

Considerations:

• While this is probably the most effort/reward effective way of solving this, "fix-by-documenting" is not really a profound solution. If there are better ideas, pleas share.
• The estimates of the minimal requirements are as good as I can judge. If more elaborate ways of measuring them are known, please share too.

References:

Issue #315.

[iwilltry42]

Thanks for adding this to the docs @Filius-Patris
I can confirm, that a single-server cluster requires at least 1GB of RAM (tested on a t2.micro instance with docker 20.10 and node-exporter running in a container).
So I guess your estimates are pretty much good to go.