Add documentation for file output (#119)

Author: camdencheek

docs/operators/file_output.md

@@ -0,0 +1,31 @@
+## `file_output` operator
+
+The `file_output` operator will write log entries to a file. By default, they will be written as JSON-formatted lines, but if a `Format` is provided, that format will be used as a template to render each log line
+
+### Configuration Fields
+
+| Field    | Default       | Description                                                                                                   |
+| ---      | ---           | ---                                                                                                           |
+| `id`     | `file_output` | A unique identifier for the operator                                                                          |
+| `path`   | required      | A path to write the entries to                                                                                |
+| `format` |               | A [go template](https://golang.org/pkg/text/template/) that will be used to render each entry into a log line |
+
+
+### Example Configurations
+
+#### Simple configuration
+
+Configuration:
+```yaml
+- type: file_output
+  path: /tmp/output.json
+```
+
+#### Custom format
+
+Configuration:
+```yaml
+- type: file_output
+  path: /tmp/output.log
+  format: "Time: {{.Timestamp}} Record: {{.Record}}\n"
+```