Update README.md

Additional Drupal installation tips

See also comments here: https://github.com/datagovuk/dgu-vagrant-puppet/issues/5

@noels can you review this as I think it is important
I don't know much about composer, but I've added instructions earlier on in the README now for how to install it, so I think this line is redundant. Also, I'm not clear how guzzle was previously installed ("wrong version") or how this line attempts to install it. Can @batje or @noels help here?