Collector documentation (#113)

* Initial draft of collector documentation

* Add docs for all collectors

* Update main README with links to docs

* Fix link syntax

docs/collector.logical_disk.md

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+# logical_disk collector
+
+The logical_disk collector exposes metrics about logical disks (in contrast to physical disks)
+
+Metric name prefix | `logical_disk`
+Classes             | [`Win32_PerfRawData_PerfDisk_LogicalDisk`](https://msdn.microsoft.com/en-us/windows/hardware/aa394307(v=vs.71))
+Enabled by default? | Yes
+
+## Flags
+
+### `--collector.logical_disk.volume-whitelist`
+
+If given, a disk needs to match the whitelist regexp in order for the corresponding disk metrics to be reported
+
+### `--collector.logical_disk.volume-blacklist`
+
+If given, a disk needs to *not* match the blacklist regexp in order for the corresponding disk metrics to be reported
+
+## Metrics
+
+Name | Description | Type | Labels
+-----|-------------|------|-------
+`requests_queued` | _Not yet documented_ | gauge | `volume`
+`read_bytes_total` | _Not yet documented_ | counter | `volume`
+`reads_total` | _Not yet documented_ | counter | `volume`
+`write_bytes_total` | _Not yet documented_ | counter | `volume`
+`writes_total` | _Not yet documented_ | counter | `volume`
+`read_seconds_total` | _Not yet documented_ | counter | `volume`
+`write_seconds_total` | _Not yet documented_ | counter | `volume`
+`free_bytes` | _Not yet documented_ | gauge | `volume`
+`size_bytes` | _Not yet documented_ | gauge | `volume`
+`idle_seconds_total` | _Not yet documented_ | counter | `volume`
+`split_ios_total` | _Not yet documented_ | counter | `volume`
+
+### Example metric
+_This collector does not yet have explained examples, we would appreciate your help adding them!_
+
+## Useful queries
+_This collector does not yet have any useful queries added, we would appreciate your help adding them!_
+
+## Alerting examples
+_This collector does not yet have alerting examples, we would appreciate your help adding them!_