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.textfile.md

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+# textfile collector
+
+The textfile collector exposes metrics from files written by other processes.
+
+Metric name prefix | `textfile`
+Classes             | None
+Enabled by default? | Yes
+
+## Flags
+
+### `--collector.textfile.directory`
+
+The directory containing the files to be ingested.
+
+Default value: `C:\Program Files\wmi_exporter\textfile_inputs`
+
+Required: No
+
+## Metrics
+
+Metrics will primarily come from the files on disk. The below listed metrics
+are collected to give information about the reading of the metrics themselves.
+
+Name | Description | Type | Labels
+-----|-------------|------|-------
+`wmi_textfile_scrape_error` | 1 if there was an error opening or reading a file, 0 otherwise | gauge | None
+`wmi_textfile_mtime_seconds` | Unix epoch-formatted mtime (modified time) of textfiles successfully read | gauge | file
+
+### 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!_