Add ipmi_sensor permission documentation

plugins/inputs/ipmi_sensor/README.md

-# Telegraf ipmi plugin
+# IPMI Sensor Input Plugin
 
-Get bare metal metrics using the command line utility `ipmitool`
-
-see ipmitool(https://sourceforge.net/projects/ipmitool/files/ipmitool/)
+Get bare metal metrics using the command line utility
+[`ipmitool`](https://sourceforge.net/projects/ipmitool/files/ipmitool/).
 
 If no servers are specified, the plugin will query the local machine sensor stats via the following command:
 
@@ -16,18 +15,7 @@ When one or more servers are specified, the plugin will use the following comman
 ipmitool -I lan -H SERVER -U USERID -P PASSW0RD sdr
 ```
 
-## Measurements
-
-- ipmi_sensor:
-
-    * Tags: `name`, `unit`
-    * Fields:
-      - status
-      - value
-
-The `server` tag will be made available when retrieving stats from remote server(s).
-
-## Configuration
+### Configuration
 
 ```toml
 # Read metrics from the bare metal servers via IPMI
@@ -52,26 +40,49 @@ The `server` tag will be made available when retrieving stats from remote server
   timeout = "20s"
 ```
 
-## Output
+### Measurements
+
+- ipmi_sensor:
+  - tags:
+    - name
+    - unit
+    - server (only when retrieving stats from remote servers)
+  - fields:
+    - status (int)
+    - value (float)
+
+
+#### Permissions
+
+When gathering from the local system, Telegraf will need permission to the
+impi device node.  When using udev you can create the device node giving
+`rw` permissions to the `telegraf` user by adding the following rule to
+`/etc/udev/rules.d/52-telegraf-ipmi.rules`:
+
+```
+KERNEL=="ipmi*", MODE="660", GROUP="telegraf"
+```
+
+### Example Output
 
 When retrieving stats from a remote server:
 ```
-> ipmi_sensor,server=10.20.2.203,unit=degrees_c,name=ambient_temp status=1i,value=20 1458488465012559455
-> ipmi_sensor,server=10.20.2.203,unit=feet,name=altitude status=1i,value=80 1458488465012688613
-> ipmi_sensor,server=10.20.2.203,unit=watts,name=avg_power status=1i,value=220 1458488465012776511
-> ipmi_sensor,server=10.20.2.203,unit=volts,name=planar_3.3v status=1i,value=3.28 1458488465012861875
-> ipmi_sensor,server=10.20.2.203,unit=volts,name=planar_vbat status=1i,value=3.04 1458488465013072508
-> ipmi_sensor,server=10.20.2.203,unit=rpm,name=fan_1a_tach status=1i,value=2610 1458488465013137932
-> ipmi_sensor,server=10.20.2.203,unit=rpm,name=fan_1b_tach status=1i,value=1775 1458488465013279896
+ipmi_sensor,server=10.20.2.203,unit=degrees_c,name=ambient_temp status=1i,value=20 1458488465012559455
+ipmi_sensor,server=10.20.2.203,unit=feet,name=altitude status=1i,value=80 1458488465012688613
+ipmi_sensor,server=10.20.2.203,unit=watts,name=avg_power status=1i,value=220 1458488465012776511
+ipmi_sensor,server=10.20.2.203,unit=volts,name=planar_3.3v status=1i,value=3.28 1458488465012861875
+ipmi_sensor,server=10.20.2.203,unit=volts,name=planar_vbat status=1i,value=3.04 1458488465013072508
+ipmi_sensor,server=10.20.2.203,unit=rpm,name=fan_1a_tach status=1i,value=2610 1458488465013137932
+ipmi_sensor,server=10.20.2.203,unit=rpm,name=fan_1b_tach status=1i,value=1775 1458488465013279896
 ```
 
 When retrieving stats from the local machine (no server specified):
 ```
-> ipmi_sensor,unit=degrees_c,name=ambient_temp status=1i,value=20 1458488465012559455
-> ipmi_sensor,unit=feet,name=altitude status=1i,value=80 1458488465012688613
-> ipmi_sensor,unit=watts,name=avg_power status=1i,value=220 1458488465012776511
-> ipmi_sensor,unit=volts,name=planar_3.3v status=1i,value=3.28 1458488465012861875
-> ipmi_sensor,unit=volts,name=planar_vbat status=1i,value=3.04 1458488465013072508
-> ipmi_sensor,unit=rpm,name=fan_1a_tach status=1i,value=2610 1458488465013137932
-> ipmi_sensor,unit=rpm,name=fan_1b_tach status=1i,value=1775 1458488465013279896
+ipmi_sensor,unit=degrees_c,name=ambient_temp status=1i,value=20 1458488465012559455
+ipmi_sensor,unit=feet,name=altitude status=1i,value=80 1458488465012688613
+ipmi_sensor,unit=watts,name=avg_power status=1i,value=220 1458488465012776511
+ipmi_sensor,unit=volts,name=planar_3.3v status=1i,value=3.28 1458488465012861875
+ipmi_sensor,unit=volts,name=planar_vbat status=1i,value=3.04 1458488465013072508
+ipmi_sensor,unit=rpm,name=fan_1a_tach status=1i,value=2610 1458488465013137932
+ipmi_sensor,unit=rpm,name=fan_1b_tach status=1i,value=1775 1458488465013279896
 ```