Updated box documentation to include `vagrant box list`

docs/boxes.md

@@ -47,6 +47,20 @@ $ vagrant box remove ubuntu_base
 **Warning:** There is no going back from this command. This command literally
 deletes the files off the filesystem.
 
+## Listing Installed Boxes
+
+Keeping track of the installed boxes can be difficult. Vagrant provides the
+`vagrant box list` command to list all installed boxes.
+
+{% highlight bash %}
+$ vagrant box list
+=====================================================================
+Installed Vagrant Boxes:
+
+base
+=====================================================================
+{% endhighlight %}
+
 <a name="creating-a-box"> </a>
 ## Creating a Box