Update changelog for release

Author: MarkBaker

CHANGELOG.md

@@ -5,7 +5,7 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com)
 and this project adheres to [Semantic Versioning](https://semver.org).
 
-## Unreleased - TBD
+## 1.18.0 - 2021-05-31
 
 ### Added
 - Enhancements to CSV Reader, allowing options to be set when using `IOFactory::load()` with a callback to set delimiter, enclosure, charset etc. [PR #2103](https://github.com/PHPOffice/PhpSpreadsheet/pull/2103) - See [documentation](https://github.com/PHPOffice/PhpSpreadsheet/blob/master/docs/topics/reading-and-writing-to-file.md#csv-comma-separated-values) for details.