Docs: Add missing date in changelog (#218)

Author: bmish

CHANGELOG.md

@@ -6,6 +6,8 @@ Generated by [`auto-changelog`](https://github.com/CookPete/auto-changelog).
 
 #### [v4.0.0](https://github.com/not-an-aardvark/eslint-plugin-eslint-plugin/compare/v4.0.0-3...v4.0.0)
 
+> 15 October 2021
+
 This release is identical to v4.0.0-3. Summary of changes:
 
 - Breaking: Enable additional `recommended` rules for v4 [`#152`](https://github.com/not-an-aardvark/eslint-plugin-eslint-plugin/pull/152)