Take a snapshot before starting.

As suggested on the wallpaper of modern.IE VMs:

> Snapshot/backup:
> Create a snapshot (or keep a backup of downloaded archive) before
first booting and working with this VM, so that you can reset quickly
after the OS trial expires.

Also changed the read-me accordingly.
Correction as per PR comment.
Merge pull request #1 from rinopo/rinopo/snapshot

Take a snapshot before starting.