Update README

What do you think about these updates to the README?

- Might be a nice tip to mention that people can adjust their sudoers file to avoid having to authenticate for NFS on every `vagrant up`. 

- Looks like the root password for mysql _is_ set now, as of #28.

- Looks like the option `run_composer` defaults to true ("Install Dependencies with Composer" task in `roles/wordpress-sites/tasks/main.yml`).