Adds instructions concerning Bundler versions

Because Vagrant requires certain versions of Bundler in order to work properly, the correct version of Bundler must be installed manually.  I gathered this information from a couple of sources.

It's handy to have this information in this specific guide :)

website/source/docs/plugins/development-basics.html.md

@@ -66,7 +66,23 @@ allows you to add multiple plugins to Vagrant for development, if
 your plugin works with another plugin.
 
 With this structure in place, your workflow should be like any other
-Ruby project. When you want to manually test your plugin, use
+Ruby project, with one exception.  Because Vagrant uses the internal
+APIs of Bundler, the version of Bundler used for Vagrant development
+is pinned to a specific range defined in the
+[vagrant.gemspec](https://github.com/mitchellh/vagrant/blob/master/vagrant.gemspec)
+file.  If the upper bound version is `1.10.6`, then install the 
+required version of Bundler and Vagrant dependencies with the
+following commands:
+
+```
+gem install bundler --version '1.10.6' && \
+bundle _1.10.6_ install
+```
+
+*Be sure to check [vagrant.gemspec](https://github.com/mitchellh/vagrant/blob/master/vagrant.gemspec)
+and use the upper bound version of Bundler*
+
+When you want to manually test your plugin, use
 `bundle exec vagrant` in order to run Vagrant with your plugin
 loaded (as we specified in the Gemfile).