Describe global and per-rule requirements

[tbooth]

Suggestion from @cmeesters to add to Chapter 9:

describe the difference between global and per-rule requirements.
A global requirement might be snakemake, the executor plugin, the file system plugin
(as a remedy to I/O contention and perhaps some remote file plugin for data management).

[tbooth]

I'm not convinced this topic fits into Episode 9 which is concerned with the basics of workflow performance. The text in the episode does try to make it clear that this here is just an introduction, and we have a very brief mention of the topic of cluster execution but there's no way we can actually cover this in any depth within the course.

I think if we were to cover requirements (and the resources: fields) this would warrant a separate topic, and would have to be done in conjunction with really getting into the details of cluster (and cloud?) execution.