Update default.config.yml add some documentation on vagrant plugins

[frob]

Add a bit of documentation around excluding vagrant plugins.

This is in conjunction with @1378

[geerlingguy]

I think it's okay as-is, as it's the same case with any other configuration value inside config.yml—installed_extras, apache_vhosts, etc. all work the same, where if you want to override, you have to copy out the entire variable then remove what you don't want, and add what you do.

[oxyc]

It's also somewhat documented here http://docs.drupalvm.com/en/latest/getting-started/configure-drupalvm/

Note: The merge of variables in these files is shallow, if you want to override a single item in a list, you will need to re-define all items in that list.