outdated installation documentation

[swalchemist]

I was led astray by https://github.com/bats-core/bats-docs, where it says to run brew tap kaos/shell when installing bats using homebrew. I found that the instructions on the https://github.com/bats-core/homebrew-bats-core README are correct, as well as https://github.com/bats-core/bats-core/wiki/Install-Bats-Using-a-Package#-mac-osx, though less complete.

Re-reading bats-docs, I see it says "This is a proposal describing how a [Bats][bats] helper library should look and behave," so perhaps it's not intended to be general installation instructions, but it still causes confusion.

Should we update the bats-docs README to avoid confusion?