add README for reconfigurator-exec-unsafe - #11243

control: job assigned to worker 01M1Q9H80AQW54WS3T32CCCJV2 [factory aws, i-00fed13842f473c40] (queued for 49 s)
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|  Documenting omicron-uuid-kinds v0.1.0 (/work/oxidecomputer/omicron/uuid-kinds)
|  Documenting dev-tools-common v0.1.0 (/work/oxidecomputer/omicron/dev-tools/common)
|     Finished `dev` profile [unoptimized + debuginfo] target(s) in 7m 34s
| warning: the following packages contain code that will be rejected by a future version of  [...]
| note: to see what the problems were, use the option `--future-incompat-report`, or run `ca [...]
|    Generated /work/oxidecomputer/omicron/target/doc/api_identity/index.html and 205 other  [...]
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| real     7:34.960027821
| user    20:27.581875545
| sys      4:22.707658784
| trap        0.298109178
| tflt        0.007592206
| dflt        0.106854684
| kflt        0.000030476
| lock  7:15:23.613339783
| slp   2:32:59.911579899
| lat        34.798380761
| stop     2:14.544545506
task: process exited: duration 840764 ms, exit code 0
|W| found 0 output files