Explain how and why to install HEAD with Homebrew

The extra instructions and information in this commit should help people like me, who were bitten by rbenv not having officially released in so long.

I was getting errors with fish shell, yet I knew I had the latest version of rbenv. I eventually figured out that fish support was added more recently than the last release, and has never made it into a release. But the README did not help – I uninstalled and reinstalled rbenv according to the README, but I still got the wrong version. This commit fixes that.

If rbenv ever changes to a faster release schedule, these instructions can be deleted.