Docs for configurable versions

@luislavena @larskanis I'm QAing https://github.com/tjschuck/rake-compiler-dev-box/pull/12 right now, but one notable thing that's missing is documentation :wink: 

This adds it.  Please let me know if it makes sense!

Check out https://github.com/tjschuck/rake-compiler-dev-box/blob/551b421bc7e5be421b33828f9ba6129f6fe17f49/README.md for the Markdown-rendered version.