Add vagrant-vbguest plugins recommendation to the README, group optional plugin steps

[fabacab]

As per the discussion in issue #800, some people find it useful to use the vagrant-vbguest plugin for Vagrant to keep VirtualBox Guest Additions versions synchronized between the host and guest. Doing so can sometimes prevent subtle shared folder errors and doesn't seem to interfere with VVV's own operation.

This commit adds that plugin as a recommended installation step to the README. This commit also groups the three recommended plugins into a single logical step called "Optionally, install some convenient Vagrant plugins." Prior to this commit, each plugin was listed as its own step in the "first vagrant up" section of the README.

[jeremyfelt]

Thanks for the PR, @meitar! I think there's an opportunity to continue improving these instructions and this is a good addition.