rbenv: add README (#8293)

plugins/rbenv/README.md

+# rbenv plugin
+
+The primary job of this plugin is to provide `rbenv_prompt_info` which can be added to your theme to include Ruby
+version and gemset information into your prompt.
+
+Some functionality of this plugin will not work unless you also have the rbenv plugin *gemset* installed.
+https://github.com/jf/rbenv-gemset
+
+To use it, add `rbenv` to the plugins array in your zshrc file:
+```zsh
+plugins=(... rbenv)
+```
+
+## Alias
+
+| Alias          | Command             | Description                      |
+|----------------|---------------------|----------------------------------|
+| rubies         | `rbenv versions`    | List the installed Ruby versions |
+| gemsets        | `rbenv gemset list` | List the existing gemsets        |
+
+## Functions
+
+* `current_ruby`: The version of Ruby currently being used.
+* `current_gemset`: The name of the current gemset.
+* `gems`: Lists installed gems with enhanced formatting and color.
+* `rbenv_prompt_info`: For adding information to your prompt. Format: `<ruby version>@<current gemset>`.