[PHP 8.3] Update README file with str_increment and str_decrement functions

Related: #453 and #452

Author: Ayesh

README.md

@@ -9,6 +9,7 @@ This component provides features added to PHP 8.3 core:
 - [`ldap_exop_sync`](https://wiki.php.net/rfc/deprecate_functions_with_overloaded_signatures)
 - [`ldap_connect_wallet`](https://wiki.php.net/rfc/deprecate_functions_with_overloaded_signatures)
 - [`stream_context_set_options`](https://wiki.php.net/rfc/deprecate_functions_with_overloaded_signatures)
+- [`str_increment` and `str_decrement`](https://wiki.php.net/rfc/saner-inc-dec-operators)
 - [`Date*Exception/Error classes`](https://wiki.php.net/rfc/datetime-exceptions)
 - [`SQLite3Exception`](https://wiki.php.net/rfc/sqlite3_exceptions)