Document how to use pod affinity

[corvus-ch]

This came up in the APPUiO Community chat: https://community.appuio.ch/channel/community?msg=vRrbnPEEoh4DEWZui.

• Pointing out, that this is a possibility and how to make use of it.
• More importantly, let users know which (node) labels are available to them (they can not access that information on the cluster due to a lack of privileges).
  • So far, it is only kubernetes.io/hostname.

[gexclaude]

Ping! I was just wondering on what I need to consider. Could you give an example config that works on appuio?