added PHP_DATE_TIMEZONE env var to readme

Author: indiegenus

README.md

@@ -65,6 +65,7 @@ Variable | Default Value | Description
 **COMPOSER_ALLOW_SUPERUSER** | `1` | Allows composer to run with super user
 **COMPOSER_MEMORY_LIMIT** | `-1` | Changes composer memory limit
 **ENABLE_XDEBUG** | `false` | Enables the Xdebug extension
+**PHP_DATE_TIMEZONE** | `UTC` | Changes timezone used by date/time functions.
 **PHP_MEMORY_LIMIT** | `256M` | Changes PHP memory limit
 **PHP_MAX_INPUT_VARS** | `1000`  | Changes how many input variables may be accepted on PHP
 **PHP_UPLOAD_MAX_FILESIZE** | `25M` | Changes PHP maximum size of an uploaded file