Fix VM installation instructions

This PR replaces openmicroscopy/ome-documentation#231. After discussion with @kennethgillen, we will edit the README in the source code for now as these instructions are primarily targeting the team developers.

Modifications include:
- conversion into Markdown
- use of relative source code hyperlinks
- minor formatting improvements (code blocks...)

See [this page](https://github.com/sbesson/openmicroscopy/tree/vm_instructions/docs/install/VM/README.md) to review.