Add Vagrant disclaimer to README.md

Hi folks, I use Vagrant to run one VM for all my development work. Usually projects install fine, but in cases like GitLab, Vagrant causes issues. This time I spent a few hours debugging before I thought to check if it was an issue with Vagrant. I'm requesting to merge a small disclaimer in the README that the GDK will not install out of the box on Vagrant. Maybe it would save others from a similar goose chase. But maybe I'm an anomaly.