Add other similar projects to README.md

vagrant-snap is amazing, and saved me a ton of time.

When I upgraded to vagrant 1.06 (or something like that) and vagrant-snap stop working, I found this derivative gist that did the job: https://gist.github.com/tombh/5142237

When I upgraded to vagrant 1.1 and it broke again, I had to rewrite it myself. 
The result is [vagrant-vbox-snapshot](https://github.com/dergachev/vagrant-vbox-snapshot).

> Note: Because Vagrant 1.1 has an new, non backwards-compatible plugin API, I decided against forking and just created another gem. But unquestionably it owes much to the awesomeness of vagrant-snap.

Perhaps the vagrant-snap's README.md could link these options solutions.