Document instrumentation data (was: Update README)

`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`
