out_websocket: Document ability to provide additional headers (fluent#1351)

See fluent/fluent-bit#8536.

Signed-off-by: Markus Thömmes <markusthoemmes@me.com>

Author: markusthoemmes

pipeline/outputs/websocket.md

@@ -9,6 +9,7 @@ The **websocket** output plugin allows to flush your records into a WebSocket en
 | Host | IP address or hostname of the target WebScoket Server | 127.0.0.1 |
 | Port | TCP port of the target WebScoket Server | 80 |
 | URI | Specify an optional HTTP URI for the target websocket server, e.g: /something | / |
+| Header | Add a HTTP header key/value pair. Multiple headers can be set. | |
 | Format | Specify the data format to be used in the HTTP request body, by default it uses _msgpack_. Other supported formats are _json_, _json\_stream_ and _json\_lines_ and _gelf_. | msgpack |
 | json\_date\_key | Specify the name of the date field in output | date |
 | json\_date\_format | Specify the format of the date. Supported formats are _double_, _epoch_, _iso8601_ (eg: _2018-05-30T09:39:52.000681Z_) and _java_sql_timestamp_ (eg: _2018-05-30 09:39:52.000681_) | double |