Add a "migration guide" to the `README.md` file telling how to migrate from volare to ciel

[mithro]

It would be great if the README.md file had a "migration guide" for people who are moving from volare to ciel.

I'm assuming that the guide is likely to be pretty simple and might just be "Use the ciel command line rather than volare and everything should still work the same." or similar.

[donn]

More or less. Also rm -rf ~/.volare to save some space.