Add minimal documentation to the diskio plugin (#2296)

* Add documentation to diskio plugin

* Update spelling, fix iops_in_progress unit

plugins/inputs/system/DISK_README.md

@@ -51,3 +51,57 @@ In this case, the host's root volume should be mounted into the container and th
 > disk,fstype=autofs,path=/net free=0i,inodes_free=0i,inodes_total=0i,inodes_used=0i,total=0i,used=0i,used_percent=0 1453832006274157077
 > disk,fstype=autofs,path=/home free=0i,inodes_free=0i,inodes_total=0i,inodes_used=0i,total=0i,used=0i,used_percent=0 1453832006274169688
 ```
+
+
+# DiskIO Input Plugin
+
+The diskio input plugin gathers metrics about disk traffic and timing.
+
+### Configuration:
+
+```
+# Read metrics about disk IO by device
+[[inputs.diskio]]
+  ## By default, telegraf will gather stats for all devices including
+  ## disk partitions.
+  ## Setting devices will restrict the stats to the specified devices.
+  # devices = ["sda", "sdb"]
+  ## Uncomment the following line if you need disk serial numbers.
+  # skip_serial_number = false
+```
+
+Data collection is based on github.com/shirou/gopsutil. This package handles platform dependencies and converts all timing information to milliseconds.
+
+
+### Measurements & Fields:
+
+- diskio
+    - reads (integer, counter)
+    - writes (integer, counter)
+    - read_bytes (integer, bytes)
+    - write_bytes (integer, bytes)
+    - read_time (integer, milliseconds)
+    - write_time (integer, milliseconds)
+    - io_time (integer, milliseconds)
+    - iops_in_progress (integer, counter) (since #2037, not yet in STABLE)
+
+### Tags:
+
+- All measurements have the following tags:
+    - name (device name)
+- If configured to use serial numbers (default: disabled):
+    - serial (device serial number)
+
+### Example Output:
+
+```
+% telegraf -config ~/.telegraf/telegraf.conf -input-filter diskio -test
+* Plugin: inputs.diskio, Collection 1
+> diskio,name=mmcblk1p2 io_time=244i,read_bytes=966656i,read_time=276i,reads=128i,write_bytes=0i,write_time=0i,writes=0i 1484916036000000000
+> diskio,name=mmcblk1boot1 io_time=264i,read_bytes=90112i,read_time=264i,reads=22i,write_bytes=0i,write_time=0i,writes=0i 1484916036000000000
+> diskio,name=mmcblk1boot0 io_time=212i,read_bytes=90112i,read_time=212i,reads=22i,write_bytes=0i,write_time=0i,writes=0i 1484916036000000000
+> diskio,name=mmcblk0 io_time=1855380i,read_bytes=135861248i,read_time=58484i,reads=4081i,write_bytes=364068864i,write_time=7128792i,writes=18019i 1484916036000000000
+> diskio,name=mmcblk0p1 io_time=1855256i,read_bytes=134915072i,read_time=58256i,reads=3958i,write_bytes=364068864i,write_time=7128792i,writes=18019i 1484916036000000000
+> diskio,name=mmcblk1 io_time=384i,read_bytes=2633728i,read_time=728i,reads=323i,write_bytes=0i,write_time=0i,writes=0i 1484916036000000000
+> diskio,name=mmcblk1p1 io_time=216i,read_bytes=860160i,read_time=288i,reads=106i,write_bytes=0i,write_time=0i,writes=0i 1484916036000000000
+```