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.service.md

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+# service collector
+
+The service collector exposes metrics about Windows Services
+
+Metric name prefix | `service`
+Classes             | [`Win32_Service`](https://msdn.microsoft.com/en-us/library/aa394418(v=vs.85).aspx)
+Enabled by default? | Yes
+
+## Flags
+
+### `--collector.service.services-where`
+
+A WMI filter on which services to include. Recommended to keep down number of returned metrics.
+
+Example: `--collector.service.services-where="Name='wmi_exporter'"`
+
+## Metrics
+
+Name | Description | Type | Labels
+-----|-------------|------|-------
+`wmi_service_state` | The state of the service, 1 if the current state, 0 otherwise | gauge | name, state
+`wmi_service_start_mode` | The start mode of the service, 1 if the current start mode, 0 otherwise | gauge | name, start_mode
+`wmi_service_status` | The status of the service, 1 if the current status, 0 otherwise | gauge | name, status
+
+For the values of the `state`, `start_mode` and `status` labels, see below.
+
+### States
+
+A service can be in the following states:
+- `stopped`
+- `start pending`
+- `stop pending`
+- `running`
+- `continue pending`
+- `pause pending`
+- `paused`
+- `unknown`
+
+### Start modes
+
+A service can have the following start modes:
+- `boot`
+- `system`
+- `auto`
+- `manual`
+- `disabled`
+
+### Status
+
+A service can have any of the following statuses:
+- `ok`
+- `error`
+- `degraded`
+- `unknown`
+- `pred fail`
+- `starting`
+- `stopping`
+- `service`
+- `stressed`
+- `nonrecover`
+- `no contact`
+- `lost comm`
+
+Note that there is some overlap with service state.
+
+### 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!_