Update README.md

Update README.md to add warning about `Breaking Change` in v1.10.0

Author: khoih-prog

README.md

@@ -11,6 +11,7 @@
 
 ## Table of Contents
 
+* [Important Breaking Change from v1.10.0](#Important-Breaking-Change-from-v1100)
 * [Why do we need this ESPAsync_WiFiManager library](#why-do-we-need-this-async-espasync_wifimanager-library)
   * [Features](#features)
   * [Why Async is better](#why-async-is-better)
@@ -145,6 +146,28 @@
 ---
 ---
 
+### Important Breaking Change from v1.10.0
+
+Please have a look at [HOWTO Fix `Multiple Definitions` Linker Error](#howto-fix-multiple-definitions-linker-error)
+
+From v1.10.0, you must use
+
+```
+#include <ESPAsync_WiFiManager.h>               //https://github.com/khoih-prog/ESPAsync_WiFiManager
+
+// To be included only in main(), .ino with setup() to avoid `Multiple Definitions` Linker Error
+#include <ESPAsync_WiFiManager-Impl.h>          //https://github.com/khoih-prog/ESPAsync_WiFiManager
+```
+
+instead of only
+
+```
+#include <ESPAsync_WiFiManager.h>               //https://github.com/khoih-prog/ESPAsync_WiFiManager
+```
+
+---
+---
+
 ### Why do we need this Async [ESPAsync_WiFiManager library](https://github.com/khoih-prog/ESPAsync_WiFiManager)
 
 #### Features
@@ -286,6 +309,7 @@ You can use
 in many files. But be sure to use the following `#include <ESPAsync_WiFiManager-Impl.h>` **in just 1 `.h`, `.cpp` or `.ino` file**, which must **not be included in any other file**, to avoid `Multiple Definitions` Linker Error
 
 ```
+// To be included only in main(), .ino with setup() to avoid `Multiple Definitions` Linker Error
 #include <ESPAsync_WiFiManager-Impl.h>          //https://github.com/khoih-prog/ESPAsync_WiFiManager
 ```