rake: add README (#8254)

plugins/rake/README.md

+# Rake plugin
+
+This plugin adds support for [rake](https://ruby.github.io/rake/), the Ruby
+build tool or Ruby Make.
+
+To use it, add `rake` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... rake)
+```
+
+## Aliases
+
+The plugin aliases the rake command so you can pass arguments when invoking rake tasks
+without having to escape the brackets, i.e., you can run
+```
+rake namespace:task['argument']
+```
+instead of having to do
+```
+rake namespace:task\['argument'\]
+```
+
+| Alias  | Command                        | Description                                   |
+|--------|--------------------------------|-----------------------------------------------|
+| rake   | `noglob rake`                  | Allows unescaped square brackets              |
+| brake  | `noglob bundle exec rake`      | Same as above but call rake using bundler     |
+| srake  | `noglob sudo rake`             | Same as rake but using sudo                   |
+| sbrake | `noglob sudo bundle exec rake` | Same as above but using both sudo and bundler |
+
+## Jim Weirich
+
+The plugin also aliases `rake` to [`jimweirich`](https://github.com/jimweirich), author of Rake
+and big time contributor to the Ruby open source community. He passed away in 2014:
+
+> Thank you Jim for everything you contributed to the Ruby and open source community 
+> over the years. We will miss you dearly. — [**@robbyrussell**](https://github.com/robbyrussell/oh-my-zsh/commit/598a9c6f990756386517d66b6bcf77e53791e905)