Php84/README.md: add mb_trim, mb_ltrim and mb_rtrim

Author: jorrit

src/Php84/README.md

@@ -8,6 +8,7 @@ This component provides features added to PHP 8.4 core:
 - [`Deprecated`](https://wiki.php.net/rfc/deprecated_attribute)
 - `CURL_HTTP_VERSION_3` and `CURL_HTTP_VERSION_3ONLY` constants
 - [`fpow`](https://wiki.php.net/rfc/raising_zero_to_power_of_negative_number)
+- [`mb_trim`, `mb_ltrim` and `mb_rtrim`](https://wiki.php.net/rfc/mb_trim)
 
 More information can be found in the
 [main Polyfill README](https://github.com/symfony/polyfill/blob/main/README.md).