Add documentation for plugins entry in Vagrantfile

website/source/docs/vagrantfile/vagrant_settings.html.md

@@ -22,6 +22,38 @@ the host. Vagrant needs to know this information in order to perform some
 host-specific things, such as preparing NFS folders if they're enabled.
 You should only manually set this if auto-detection fails.
 
+`config.vagrant.plugins` - (string, array, hash) - Define plugin, list of
+plugins, or definition of plugins to install for the local project. Vagrant
+will require these plugins be installed and available for the project. If
+the plugins are not available, it will attempt to automatically install
+them into the local project. When requiring a single plugin, a string can
+be provided:
+
+```ruby
+config.vagrant.plugins "vagrant-plugin"
+```
+
+If multiple plugins are required, they can be provided as an array:
+
+```ruby
+config.vagrant.plugins ["vagrant-plugin", "vagrant-other-plugin"]
+```
+
+Plugins can also be defined as a Hash, which supports setting extra options
+for the plugins. When a Hash is used, the key is the name of the plugin, and
+the value is a Hash of options for the plugin. For example, to set an explicit
+version of a plugin to install:
+
+```ruby
+config.vagrant.plugins {"vagrant-scp" => {"version" => "1.0.0"}}
+```
+
+Supported options are:
+
+* `entry_point` - Path for Vagrant to load plugin
+* `sources` - Custom sources for downloading plugin
+* `version` - Version constraint for plugin
+
 `config.vagrant.sensitive` - (string, array) - Value or list of values that
 should not be displayed in Vagrant's output. Value(s) will be removed from
 Vagrant's normal UI output as well as logger output.