Installation instructions

* Recommend Debian/Ubuntu users install the [ruby-full](https://packages.debian.org/stable/ruby-full) meta-package.
* Organized the installation instructions by method (package manager, installer, manager, build from source) then by platform/tool.
* Added [ruby-build](https://github.com/sstephenson/ruby-build#readme) and [ruby-install](https://github.com/postmodern/ruby-install#readme) to the Installers list.
* Added [chruby](https://github.com/postmodern/chruby#readme) to the Managers section.