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.netframework_clrinterop.md

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+# netframework_clrinterop collector
+
+The netframework_clrinterop collector exposes metrics about interop between the dotnet framework and outside components.
+
+Metric name prefix | `netframework_clrinterop`
+Classes             | `Win32_PerfRawData_NETFramework_NETCLRInterop`
+Enabled by default? | No
+
+## Flags
+
+None
+
+## Metrics
+
+Name | Description | Type | Labels
+-----|-------------|------|-------
+`wmi_...` | ... | counter/gauge/histogram/summary | ...
+`wmi_netframework_clrinterop_com_callable_wrappers_total` | Displays the current number of COM callable wrappers (CCWs). A CCW is a proxy for a managed object being referenced from an unmanaged COM client. | counter | `process`
+`wmi_netframework_clrinterop_interop_marshalling_total` | Displays the total number of times arguments and return values have been marshaled from managed to unmanaged code, and vice versa, since the application started. | counter | `process`
+`wmi_netframework_clrinterop_interop_stubs_created_total` | Displays the current number of stubs created by the common language runtime. Stubs are responsible for marshaling arguments and return values from managed to unmanaged code, and vice versa, during a COM interop call or a platform invoke call. | counter | `process`
+
+### 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!_