uncapitalize several doc files

* `alternate_cask_taps.md`
* `automation.md`
* `hacking.md`
* `releasing.md`

Longstanding convention demands that certain documents such as `README.md` be in all caps, and GitHub treats those files specially.  However, there was no need for us to follow that pattern with every doc file, and currently it keeps the truly important ones from standing out.

I'd like to have downcased `CASK_LANGUAGE_REFERENCE.md` as well, but that would break too many links, and I suppose it is rather more important than its siblings.