output: file: add documentation for out_file

Signed-off-by: Takahiro YAMASHITA <nokute78@gmail.com>

Author: nokute78

SUMMARY.md

@@ -49,6 +49,7 @@
     * [Kubernetes](filter/kubernetes.md)
 * [Output Plugins](output/README.md)
     * [Elasticsearch](output/elasticsearch.md)
+    * [File](output/file.md)
     * [FlowCounter](output/flowcounter.md)
     * [Forward](output/forward.md)
     * [HTTP](output/http.md)

output/README.md

@@ -4,7 +4,8 @@ The _output plugins_ defines where [Fluent Bit](http://fluentbit.io) should flus
 
 | name                      |  title             | description     |
 |---------------------------|--------------------|-----------------|
-| [flowcounter](flowcounter.md)| FlowCounter     | Count records   |
+| [file](file.md)           | File               | flush records to a file. |
+| [flowcounter](flowcounter.md)| FlowCounter     | Count records.   |
 | [forward](forward.md)     | Forward  | Fluentd forward protocol. |
 | [http](http.md)           | HTTP     | Flush records to an HTTP end point. |
 | [influxdb](influxdb.md)   | InfluxDB | Flush records to InfluxDB time series database. |

output/file.md

@@ -0,0 +1,38 @@
+# File
+
+The __file__ output plugin allows to write the data received through the _input_ plugin to file. 
+
+## Configuration Parameters
+
+The plugin supports the following configuration parameters:
+
+| Key  | Description |
+|------|-------------|
+| Path | File path to output. If not set, the filename will be tag name.|
+
+## Getting Started
+
+You can run the plugin from the command line or through the configuration file:
+
+### Command Line
+
+From the command line you can let Fluent Bit count up a data with the following options:
+
+```bash
+$ fluent-bit -i cpu -o file -p path=output.txt
+```
+
+### Configuration File
+
+In your main configuration file append the following Input & Output sections:
+
+```python
+[INPUT]
+    Name cpu
+    Tag  cpu
+
+[OUTPUT]
+    Name file
+    Match *
+    Path output.txt
+```