README instructions for experiment.

experiment/README.md

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+# Quickstart
+
+Everything is packaged into a single `do_all.sh` script that takes the
+name of the machine to run on. E.g.:
+
+```bash
+./experiment/do_all.sh sapling
+```
+
+See the contents of `do_all.sh` to look at the individual steps.
+
+This will launch a set of jobs into the batch system on the machine,
+check `slurm-*.out` for output from the specific runs.
+
+Note: this will download Legion if it is not already present, but will
+not update Legion. Any existing Legion will be reconfigured/rebuilt.