Added example to determine interface inside callbacks (me-no-dev#446)

Author: nouser2013

README.md

@@ -70,6 +70,7 @@ To use this library you might need to have the latest git versions of [ESP32](ht
     - [Serve different site files in AP mode](#serve-different-site-files-in-ap-mode)
     - [Rewrite to different index on AP](#rewrite-to-different-index-on-ap)
     - [Serving different hosts](#serving-different-hosts)
+    - [Determine interface inside callbacks](#determine-interface-inside-callbacks)
   - [Bad Responses](#bad-responses)
     - [Respond with content using a callback without content length to HTTP/1.0 clients](#respond-with-content-using-a-callback-without-content-length-to-http10-clients)
   - [Async WebSocket Plugin](#async-websocket-plugin)
@@ -885,6 +886,18 @@ server.serveStatic("/", SPIFFS, "/host1/").setFilter(filterOnHost1);
 server.serveStatic("/", SPIFFS, "/www/");
 ```
 
+### Determine interface inside callbacks
+```cpp
+  String RedirectUrl = "http://";
+  if (ON_STA_FILTER(request)) {
+    RedirectUrl += WiFi.localIP().toString();
+  } else {
+    RedirectUrl += WiFi.softAPIP().toString();
+  }
+  RedirectUrl += "/index.htm";
+  request->redirect(RedirectUrl);
+```
+
 ## Bad Responses
 Some responses are implemented, but you should not use them, because they do not conform to HTTP.
 The following example will lead to unclean close of the connection and more time wasted