Added another step (mmistakes#2294)

The step is required to get the portfolio collection to work.

docs/_docs/13-collections.md

@@ -41,6 +41,19 @@ defaults:
       share: true
 ```
 
+Now make a portfolio.md file in the '_pages' folder.
+
+```yaml
+---
+title: Portfolio
+layout: collection
+permalink: /portfolio/
+collection: portfolio
+entries_layout: grid
+classes: wide
+---
+```
+
 And then create portfolio content like [`_portfolio/foo-bar-website.md`](https://github.com/{{ site.repository }}/blob/master/docs/_portfolio/foo-bar-website.md), to end up with something like this.
 
-![portfolio collection example]({{ "/assets/images/mm-portfolio-collection-example.jpg" | relative_url }})
+![portfolio collection example]({{ "/assets/images/mm-portfolio-collection-example.jpg" | relative_url }})