Improve accessibility of navigation menu button

Add `type` and label text.

Fixes #1099

docs/_docs/18-history.md

@@ -4,7 +4,7 @@ permalink: /docs/history/
 excerpt: "Change log of enhancements and bug fixes made to the theme."
 sidebar:
   nav: docs
-last_modified_at: 2017-10-04T08:07:18-04:00
+last_modified_at: 2017-10-04T11:06:15-04:00
 ---
 
 ## Unreleased
@@ -16,6 +16,7 @@ last_modified_at: 2017-10-04T08:07:18-04:00
 - Add note about Staticman GitHub compatibility. [#1273](https://github.com/mmistakes/minimal-mistakes/issues/1273)
 - Add missing Brazilian Portuguese translations to `ui-text.yml`. [#1278](https://github.com/mmistakes/minimal-mistakes/pull/1278)
 - Update font stack documentation. [#1292](https://github.com/mmistakes/minimal-mistakes/pull/1292)
+- Improve accessibility of navigation menu button. [#1099](https://github.com/mmistakes/minimal-mistakes/issues/1099)
 
 ### Bug Fixes