add README for this feature

README.md

@@ -265,6 +265,7 @@ Configure frps same as above.
   plugin_crt_path = ./server.crt
   plugin_key_path = ./server.key
   plugin_host_header_rewrite = 127.0.0.1
+  plugin_header_X-From-Where = frp
   ```
 
 2. Visit `https://test.yourdomain.com`.

README_zh.md

@@ -270,6 +270,7 @@ frps 的部署步骤同上。
   plugin_crt_path = ./server.crt
   plugin_key_path = ./server.key
   plugin_host_header_rewrite = 127.0.0.1
+  plugin_header_X-From-Where = frp
   ```
 
 2. 通过浏览器访问 `https://test.yourdomain.com` 即可。