Added another step (mmistakes#2294)

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

docs/_docs/13-collections.md

@@ -0,0 +1,59 @@
+---
+title: "Working with Collections"
+permalink: /docs/collections/
+excerpt: "Suggestions and Front Matter defaults for working with collections."
+last_modified_at: 2018-03-20T16:00:02-04:00
+---
+
+Collections like posts and pages work as you'd expect. If you're new to them be sure to read [Jekyll's documentation](https://jekyllrb.com/docs/collections/).
+
+The theme has been built with collections in mind and you will find [several examples]({{ "/collection-archive/" | relative_url }}) on the demo site ([portfolio]({{ "/portfolio/" | relative_url }}), [recipes]({{ "/recipes/" | relative_url }}), [pets]({{ "/pets/" | relative_url }})). 
+
+**Collections in the Wild:** This set of documentation is also [built as a collection](https://github.com/{{ site.repository }}/blob/master/docs/_docs/) if you're looking for a fully fleshed out example to inspect.
+{: .notice--info}
+
+---
+
+A popular use case for collections is to build a portfolio section as part of one's personal site. Let's quickly walk through the steps to do that.
+
+**Step 1:** Configure the portfolio collection by adding the following to `_config.yml`.
+
+```yaml
+collections:
+  portfolio:
+    output: true
+    permalink: /:collection/:path/
+```
+
+These settings essentially say output `index.html` files for each portfolio document in `_portfolio` at `_site/portfolio/<document-filename>/`.
+
+Just like posts and pages you'll probably want to set some defaults for the Front Matter:
+
+```yaml
+defaults:
+  # _portfolio
+  - scope:
+      path: ""
+      type: portfolio
+    values:
+      layout: single
+      author_profile: false
+      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 }})