jruby: add README (#8190)

plugins/jruby/README.md

+# JRuby plugin
+
+This plugin adds aliases for [JRuby](https://www.jruby.org/).
+
+To use it, add `jruby` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... jruby)
+```
+
+## Requirements
+
+This plugin assumes you already have jruby installed and available in your [path](https://www.jruby.org/getting-started).
+
+## Aliases
+
+| Alias        | Command                                                          |
+| ------------ | ---------------------------------------------------------------- |
+| `jrspec`     | `jruby --debug -S rspec --debug`                                 |
+| `jprofile`   | `jruby --profile.api -S rspec`                                   |
+| `jexec`      | `jruby -S`                                                       |