Upgrade 2-3 docs, include note about getting one single collection

Yay, @pathawks! https://github.com/jekyll/jekyll/issues/4392#issuecomment-174369983

site/_docs/history.md

@@ -10,6 +10,7 @@ permalink: "/docs/history/"
 ### Minor Enhancements
 {: #minor-enhancements-v3-1-0}
 
+- Update the Code of Conduct to the latest version ([#4402]({{ site.repository }}/issues/4402))
 - Use `Liquid::Drop`s instead of `Hash`es in `#to_liquid` ([#4277]({{ site.repository }}/issues/4277))
 - Add 'sample' Liquid filter Equivalent to Array#sample functionality ([#4223]({{ site.repository }}/issues/4223))
 - Cache parsed include file to save liquid parsing time. ([#4120]({{ site.repository }}/issues/4120))

site/_docs/upgrading/2-to-3.md

@@ -32,6 +32,16 @@ A simple conversion must be made in your templates:
 
 When iterating over `site.collections`, ensure the above conversions are made.
 
+For `site.collections.myCollection` in Jekyll 2, you now do:
+
+{% highlight liquid %}
+{% raw %}
+{% assign myCollection = site.collections | where: "label", "myCollection" | first %}
+{% endraw %}
+{% endhighlight %}
+
+This is a bit cumbersome at first, but is easier than a big `for` loop.
+
 ### Dropped dependencies
 
 We dropped a number of dependencies the Core Team felt were optional. As such, in 3.0, they must be explicitly installed and included if you use any of the features. They are: