Add notice for fixing Unknown tag 'include_cached'

Fixes #1875

docs/_docs/01-quick-start-guide.md

@@ -2,7 +2,7 @@
 title: "Quick-Start Guide"
 permalink: /docs/quick-start-guide/
 excerpt: "How to quickly install and setup Minimal Mistakes for use with GitHub Pages."
-last_modified_at: 2018-07-19T11:31:38-04:00
+last_modified_at: 2018-10-04T20:15:56-04:00
 redirect_from:
   - /theme-setup/
 toc: true
@@ -19,6 +19,9 @@ If you're running Jekyll v3.5+ and self-hosting you can quickly install the them
 **ProTip:** Be sure to remove `/docs` and `/test` if you forked Minimal Mistakes. These folders contain documentation and test pages for the theme and you probably don't want them littering up your repo.
 {: .notice--info}
 
+**Note:** The theme uses the [jekyll-include-cache](https://github.com/benbalter/jekyll-include-cache) plugin which will need to be installed in your `Gemfile` and added to the `plugins` array of `_config.yml`. Otherwise you'll throw `Unknown tag 'include_cached'` errors at build.
+{: .notice--warning}
+
 ### Ruby Gem Method
 
 Add this line to your Jekyll site's `Gemfile`: