Add docs for ipmistat collector

collectors/ipmiMetric.md

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+
+## `ipmistat` collector
+
+```json
+  "ipmistat": {
+    "ipmitool_path": "/path/to/ipmitool",
+    "ipmisensors_path": "/path/to/ipmi-sensors",
+  }
+```
+
+The `ipmistat` collector reads data from `ipmitool` (`ipmitool sensor`) or `ipmi-sensors` (`ipmi-sensors --sdr-cache-recreate --comma-separated-output`). 
+
+The metrics depend on the output of the underlying tools but contain temperature, power and energy metrics.
+
+
+