"Working on a cluster" -- nodes may be heterogeneous

[reid-a]

In the "Working on a cluster" episode, where contexts and scopes within the cluster are introduced, it's worth pointing out that the cluster's node collection may be heterogeneous. It's probably overkill to get into resource requests, but a remark that nodes are not always all equal would be valuable here.

[psteinb]

I think this is a prime candidate for a callout section. Feel free to submit a PR on this, please.