system: add explanation config_include_dir feature

See fluent/fluentd#4893

Signed-off-by: Kentaro Hayashi <[email protected]>

Author: kenhys

deployment/system-config.md

@@ -178,6 +178,18 @@ Parses config values strictly. Invalid numerical or boolean values are not allow
 
 Force disable the shared socket which is for listening a same port across multiple worker processes. When the shared socket is enabled \(it's the default behavior\), a socket is always created to enable workers to communicate with the supervisor. On Windows, it consumes a dynamic \(a.k.a ephemeral\) TCP port. If you don't prefer it, set this option as `true`. When it's disabled, you may not use plugins that listen a port such as in\_forward, in\_http and in\_syslog.
 
+### `config_include_dir`
+
+| type | default | version |
+| :--- | :--- | :--- |
+| string | "/etc/fluent/conf.d" | 1.19.0 |
+
+Specifies the additional include directory which is enabled by default.
+If there is any configuration file under `config_include_dir` directory,
+it will be loaded without specifying `@include` directive.
+
+If you want to disable this auto-load feature, set `config_include_dir ""` explicitly.
+
 ### `enable_jit` (experimental)
 
 | type | default | version |