Update CHANGELOG and history

Author: mmistakes

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ### Enhancements
 
+- Document head and footer `custom.html` includes. [#2815](https://github.com/mmistakes/minimal-mistakes/pull/2815)
 - Color notices based on skin colors instead of fixed values. [#2887](https://github.com/mmistakes/minimal-mistakes/pull/2887)
 - Add configurable datetime format. [#2844](https://github.com/mmistakes/minimal-mistakes/pull/2844)
 - Add Baidu site verfication [#2830](https://github.com/mmistakes/minimal-mistakes/pull/2830)

docs/_docs/18-history.md

@@ -5,14 +5,15 @@ permalink: /docs/history/
 excerpt: "Change log of enhancements and bug fixes made to the theme."
 sidebar:
   nav: docs
-last_modified_at: 2021-04-13T16:01:10-04:00
+last_modified_at: 2021-04-13T16:11:58-04:00
 toc: false
 ---
 
 ## Unreleased
 
 ### Enhancements
 
+- Document head and footer `custom.html` includes. [#2815](https://github.com/mmistakes/minimal-mistakes/pull/2815)
 - Color notices based on skin colors instead of fixed values. [#2887](https://github.com/mmistakes/minimal-mistakes/pull/2887)
 - Add configurable datetime format. [#2844](https://github.com/mmistakes/minimal-mistakes/pull/2844)
 - Add Baidu site verfication [#2830](https://github.com/mmistakes/minimal-mistakes/pull/2830)