Place a system metadata stamp on readme chart

[scotts]

This also reorients the data file itself to be structured as:

{
  "experiments":
    [
      # list of individual experiments
    ]
  "system_metadata":
    {
      # key-value pairs of metadata
    }
}

Because this changes the data file, I'll wait until the current experiments are done, and then I'll merge things manually.

[ahmadsharif1]

nit: why not call these results?

[scotts]

When reading a data file, I find it more clear to see "experiments" and "system_metadata". But in the code, I think "results" is more clear that it's the return value of running the experiments.