ruby: add README (#7212)

plugins/ruby/README.md

+# Ruby plugin
+
+This plugin adds aliases for common commands used in dealing with [Ruby](https://www.ruby-lang.org/en/) and [gem packages](https://rubygems.org/).
+
+To use it, add `ruby` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... ruby)
+```
+
+## Aliases
+
+| Alias | Command                                | Description                                          |
+|-------|----------------------------------------|------------------------------------------------------|
+| rb    | `ruby`                                 | The Ruby command                                     |
+| sgem  | `sudo gem`                             | Run sudo gem on the system ruby, not the active ruby |
+| rfind | `find . -name "*.rb" \| xargs grep -n` | Find ruby file                                       |
+| gin   | `gem install`                          | Install a gem into the local repository              |
+| gun   | `gem uninstall`                        | Uninstall gems from the local repository             |
+| gli   | `gem list`                             | Display gems installed locally                       |