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.cpu.md

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+# cpu collector
+
+The cpu collector exposes metrics about CPU usage
+
+Metric name prefix | `cpu`
+Classes             | [`Win32_PerfRawData_PerfOS_Processor`](https://msdn.microsoft.com/en-us/library/aa394317(v=vs.90).aspx)
+Enabled by default? | Yes
+
+## Flags
+
+None
+
+## Metrics
+
+Name | Description | Type | Labels
+-----|-------------|------|-------
+`wmi_cpu_cstate_seconds_total` | Time spent in low-power idle states | counter | `core`, `state`
+`wmi_cpu_time_total` | Time that processor spent in different modes (idle, user, system, ...) | counter | `core`, `mode`
+`wmi_cpu_interrupts_total` | Total number of received and serviced hardware interrupts | counter | `core`
+`wmi_cpu_dpcs_total` | Total number of received and serviced deferred procedure calls (DPCs) | counter | `core`
+
+### 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!_