yarn: add README

plugins/yarn/README.md

+# Yarn plugin
+
+This plugin adds completion for the [Yarn package manager](https://yarnpkg.com/en/),
+as well as some aliases for common Yarn commands.
+
+To use it, add `yarn` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... yarn)
+```
+
+## Aliases
+
+| Alias | Command                                   | Description                                                 |
+|-------|-------------------------------------------|-------------------------------------------------------------|
+| y     | `yarn`                                    | The Yarn command                                            |
+| ya    | `yarn add`                                | Install a package in dependencies (`package.json`)          |
+| yad   | `yarn add --dev`                          | Install a package in devDependencies (`package.json`)       |
+| yap   | `yarn add --peer`                         | Install a package in peerDependencies (`package.json`)      |
+| yb    | `yarn build`                              | Run the build script defined in `package.json`              |
+| ycc   | `yarn cache clean`                        | Clean yarn's global cache of packages                       |
+| ygu   | `yarn global upgrade`                     | Upgrade packages installed globally to their latest version |
+| yh    | `yarn help`                               | Show help for a yarn command                                |
+| yin   | `yarn install`                            | Install dependencies defined in `package.json`              |
+| yls   | `yarn list`                               | List installed packages                                     |
+| yout  | `yarn outdated`                           | Check for outdated package dependencies                     |
+| yrm   | `yarn remove`                             | Remove installed packages                                   |
+| yrun  | `yarn run`                                | Run a defined package script                                |
+| yst   | `yarn start`                              | Run the start script defined in `package.json`              |
+| yt    | `yarn test`                               | Run the test script defined in `package.json`               |
+| yuc   | `yarn global upgrade && yarn cache clean` | Upgrade global packages and clean yarn's global cache       |
+| yui   | `yarn upgrade-interactive`                | Prompt for which outdated packages to upgrade               |
+| yup   | `yarn upgrade`                            | Upgrade packages to their latest version                    |