Update README to clarify virtual host docroots

With the previous wording I got confused and expected Vagrant to create the
docroot directory for me. This wording will help clarify this for future users.
Merge pull request #66 from captbaritone/docroot-readme-update

Confusing wording in the README regarding docroot
Merge branch 'master' of https://github.com/r8/vagrant-lamp

# By Jordan Eldredge
# Via Jordan Eldredge (1) and Sergey Storchay (1)
* 'master' of https://github.com/r8/vagrant-lamp:
  Update README to clarify virtual host docroots