gem: add README (#7224)

plugins/gem/README.md

+# Gem plugin
+
+This plugin adds completions and aliases for [Gem](https://rubygems.org/). The completions include the common `gem` subcommands as well as the installed gems in the current directory.
+
+To use it, add `gem` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... gem)
+```
+
+## Aliases
+
+| Alias                | Command                       | Description                                |
+|----------------------|-------------------------------|--------------------------------------------|
+| gemb                 | `gem build *.gemspec`         | Build a gem from a gemspec                 |
+| gemp                 | `gem push *.gem`              | Push a gem up to the gem server            |
+| gemy [gem] [version] | `gem yank [gem] -v [version]` | Remove a pushed gem version from the index |