Add HTTP API document.

HTTPAPI.md

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+# Features
+
+* Check current status (on/off)
+
+- Toggle the client
+
+- Get server list
+
+- Switch server
+
+- Get current mode
+
+- Switch mode
+
+# Specification
+
+- #### Check current status (on/off)  `GET /status`
+
+###### Sample Return
+
+```
+{
+    "enable": true
+}
+```
+
+- #### Toggle the client  `POST /toggle`
+
+###### Sample Return
+
+```
+{
+    "status": 1
+    // 1 for toggle succeed, 0 for fail  
+}
+```
+
+- #### Get server list  `GET /servers`
+
+###### Sample Return
+
+```
+[
+	{
+		"active": 1,
+   		"id": "93C547E0-49C9-1234-9CAD-EE8D5C4A1A8F",
+    	"remark": "us1",
+		// remark: as in Server Preferences Panel of the app.
+	},
+	{
+    	"active" : 0,
+    	"id" : "71552DCD-B298-495E-904E-82DA4B07AEF8",
+    	"remark" : "hk2"
+  	},
+  	{
+    	"active" : 0,
+    	"id" : "E8879F3D-95AE-4714-BC04-9B271C2BC52D",
+    	"remark" : "jp1"
+  	},...
+]
+```
+
+- #### Switch server  `POST /servers`
+
+###### Argument
+
+| Name | Description                   | Sample                                 |
+| ---- | ----------------------------- | -------------------------------------- |
+| id   | As returned in `GET /servers` | "E8879F3D-95AE-4714-BC04-9B271C2BC52D" |
+
+###### Sample Return
+
+```
+{
+    "status": 1
+    // 1 for succeed, 0 for fail
+}
+```
+
+If the `id` is invalid or fail to match any `id` in config, "status" = 0. 
+
+- #### Get current mode  `GET /mode`
+
+###### Sample Return
+
+```
+{
+    "mode": "auto"
+}
+```
+
+ `mode`∈ {"auto", "global", "manual"}.
+
+- #### Switch mode  `POST /mode`
+
+###### Sample Return
+
+```
+{
+    "status": 1
+    // 1 for succeed, 0 for fail  
+}
+```
+
+If the `mode`∉ {"auto", "global", "manual"}, "status" = 0.