docs: rqe-optimizer README (data, results, how to use them)

[zzylol]

Adds rqe-optimizer/README.md, a guide to the folder's data and results:

• Layout: which results directory is current (autosketch-vs-asap-hydra) and which are superseded (eval: AutoSketch vs. ASAP on the synthetic mixed set (cost by use; frontier and SLA versions) #138's -synthetic and -synthetic-sla).
• Result files: file naming, top-level keys, and every field of a plan entry, with a snippet for each method's cheapest plan.
• Plans: the gzipped chosen fields, and how to turn them into deployments.
• Inputs: workload tables (RQE fields), accuracy curves, the cost table, hydra_saturation.csv.
• Usage: reproduce, run a new workload (and when a new saturation study is needed), workload_scenarios, end-to-end use.

Docs only.

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