Add documentation for clash_api

docs/configuration/experimental.md

@@ -0,0 +1,39 @@
+### Structure
+
+```json
+{
+  "experimental": {
+    "clash_api": {
+      "external_controller": "127.0.0.1:9090",
+      "external_ui": "folder",
+      "secret": ""
+    }
+  }
+}
+```
+
+### Clash API Fields
+
+!!! error ""
+
+    Clash API is not included by default, see [Installation](/#Installation).
+
+!!! note ""
+
+    Traffic statistics and connection management will disable TCP splice in linux and reduce performance, use at your own risk.
+
+#### external_controller
+
+RESTful web API listening address. Disabled if empty.
+
+#### external_ui
+
+A relative path to the configuration directory or an absolute path to a
+directory in which you put some static web resource. Clash core will then
+serve it at `http://{{external-controller}}/ui`.
+
+#### secret
+
+Secret for the RESTful API (optional)
+Authenticate by spedifying HTTP header `Authorization: Bearer ${secret}`
+ALWAYS set a secret if RESTful API is listening on 0.0.0.0