chruby: add README (#8238)

plugins/chruby/README.md

+# chruby plugin
+
+This plugin loads [chruby](https://github.com/postmodern/chruby), a tool that changes the
+current Ruby version, and completion and a prompt function to display the Ruby version.
+Supports brew and manual installation of chruby.
+
+To use it, add `chruby` to the plugins array in your zshrc file:
+```zsh
+plugins=(... chruby)
+```
+
+## Usage
+
+If you'd prefer to specify an explicit path to load chruby from
+you can set variables like so:
+
+```
+zstyle :omz:plugins:chruby path /local/path/to/chruby.sh
+zstyle :omz:plugins:chruby auto /local/path/to/auto.sh
+```