Added instructions for using Vagrant

(Paired with PR #121)
Merge branch '119_document_vagrant_build' of https://github.com/squaremo/weave into squaremo-119_document_vagrant_build
cosmetic
Elaborated on vagrant instructions

It made different assumptions to the "regular" instructions (namely,
that the reader already had a working directory).
Merge pull request #124 from squaremo/119_document_vagrant_build

Added instructions for using Vagrant