Updated README.md

Resolving the issue of unmatched VirtualBox Guest Additions requires
the installation of vagrant-vbguest plugin prior to setup and no
further changes to configuration. Hence, the README.md is updated with
a small section with regards to vagrant-vbguest.

Minor changes to the README.md was also made to enhance the
instructions.
Merge pull request #4 from prixde/Enhancement/Issue3/Non-Matching-Virtualbox-Guest-Additions

Updated README.md