Document instrumentation data (was: Update README)

[snim2]

README.md has fallen behind the code. The keys in Krun results files are currently:

• audit
• core_cycles_data
  • mperf_data
  • eta_estimates
  • data
  • starting_temperatures
  • reboots
  • aperf_data
  • instr_data
  • config
  • error_flag

[snim2]

And they have changed again! This isn't urgent but we should probably resolve this before we submit.

[snim2]

PR #302 covered the JSON keys that are still in use. The instr_data directory has not yet been described.

[vext01]

Relevant: #359