Add explanation of partials to _errors.md

source/includes/_errors.md

 # Errors
 
+<aside class="notice">This error section is stored in a separate file in `includes/_errors.md`. Slate allows you to optionally separate out your docs into many files...just save them to the `includes` folder and add them to the top of your `index.md`'s frontmatter. Files are included in the order listed.</aside>
+
 The Kittn API uses the following error codes: