Complete section on resource management + costs

[yuvipanda]

This section would focus on how to manage costs, think about costs and control costs.

• Factors affecting cost & how to control them
  • Number of machines in use (http://zero-to-jupyterhub.readthedocs.io/en/latest/user-resources.html)
    • Increase & decrease cluster size
  • Size of machines in use (http://zero-to-jupyterhub.readthedocs.io/en/latest/cost.html)
    • How to pick what suits you, and changing it up and down
  • Persistent storage per-user
  • Backups
  • Container image storage
  • How long idle servers are kept alive
• Monitoring resource usage
  • Stackdriver for monitoring user's resource usage
  • The billing console for figuring out what you are paying for
• Tearing down a cluster completely

[choldgraf]

I updated a couple of the to-do items here, but I think we have a few others to fill in if we still want to.

One question I have: some of this starts to get into more provider-specific features (e.g. stackdriver). Do we want to provide this kind of information, or keep it at the general level of "these are the things you should think about" and link out to other information for details?

[consideRatio]

I'll close this and open a 0.8 milestone issue regarding cost estimations where the main concern is recommendations about how much memory vs cpu to request / limit and considerations about autoscaling.