Add Gem README

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               |
+|-------|-----------------------|
+| gemb  | `gem build *.gemspec` |
+| gemp  | `gem push *.gem`      |
+| gemy  | `gem yank $1 -v $2`   |
+