Recommend `setup.sh` as the default installation method.

[ragaskar]

This would've saved me a few minutes of wondering why chruby was "command not found" after compiling it on my system. 😄

I mean, my fault for not fully reading the instructions, but I suspect that's how many of us operate. 😄

EDIT: Oops, also wanted to take a moment to say a big "THANK YOU" for chruby. I have used it often over the years.

[postmodern]

The reason why make install does not also install configuration is I don't want to choose for the user how chruby is configured/loaded. Users can choose whether chruby is loaded via /etc/profile.d/ or in ~/.bashrc/~/.zshrc.

[ragaskar]

Yes, that makes sense to me. Sorry if the PR wording is unclear: I'm proposing a change that would guide most users towards running setup.sh vs running make install. I suspect folks who want to customize how chruby is installed will read the whole doc, whereas users who just want a chruby asap would be better served by just running the ./setup.sh script.