Document requirements better

[lars-t-hansen]

There is a short section in the README.md that documents "early goals" and "later goals". I think we should probably expand this. In particular:

• running on non-slurm systems is a hard requirement, another reason not to depend on slurm data (unlike eg TrailblazingTurtle)
• being flexible and expandable is a hard requirement, given the variety of hardware out there (so, we could do better in terms of how we handle GPUs maybe, but it motivates the data format)
• (probably several more things that have become requirements as the code has expanded)

This is on my mind now as I am writing a requirements document for the monitoring components of the NAIC project (of which sonar is currently one of several).