add comment about --with-external-libcrypt

Author: remicollet

README.md

@@ -35,6 +35,8 @@ See the Linux man pages or [PHP Documentation](https://www.php.net/xpass)
 
 Compatible with PHP 8.0 or greater.
 
+It is recommended to ensure PHP is built using `--with-external-libcrypt` option.
+
 You need the Extended crypt library development files (libxcrypt-devel, libcrypt-dev)
 version 4.4 or greater.