Link to rbenv/chruby

Similar to 4a5099c5

doc/prepare.md

@@ -21,10 +21,13 @@ during installation.
 1. A non-root Unix user, this can be your normal user but **DO NOT** run the
    installation as a root user
 2. Ruby 2.3 (2.3.1 or newer) installed with a Ruby version manager
-   ([RVM](https://rvm.io/), rbenv, chruby, etc.), **DO NOT** use the
+   ([RVM](https://rvm.io/), [rbenv], [chruby], etc.), **DO NOT** use the
    system Ruby
 3. Bundler, which you can install with `gem install bundler`
+[rbenv]: https://github.com/rbenv/rbenv
+[chruby]: https://github.com/postmodern/chruby
+
 ### OS X 10.9 (Mavericks), 10.10 (Yosemite), 10.11 (El Capitan), macOS 10.12 (Sierra)
 Please read [the prerequisites for all platforms](#prerequisites-for-all-platforms).