add example

docs/20_Usage.md

@@ -87,6 +87,14 @@ $aUser = $oLdap->getUserInfo("john@example.com", ["memberof", "uid"]);
 print_r($oLdap->normalizeSearchentry($aUser));
 ```
 
+### Verify user and password for login
+
+```php
+// set values from $_FORM or $_POST data of your login form here
+// The variable $bAuthenticated is true if authentication of the user was successful.
+$bAuthenticated=oLdap->verifyPassword($sUser, $sPassword);
+```
+
 ### Example: search
 
 When using special chars in search then you can sanitize the search string.