composer: add README (#7291)

plugins/composer/README.md

+# composer
+
+This plugin provides completion for [composer](https://getcomposer.org/), as well as aliases
+for frequent composer commands. It also adds Composer's global binaries to the PATH, using
+Composer if available.
+
+To use it add `composer` to the plugins array in your zshrc file.
+
+```zsh
+plugins=(... composer)
+```
+
+## Aliases
+
+| Alias  | Command                                      | Description                                                                            |
+| ------ | -------------------------------------------- | -------------------------------------------------------------------------------------- |
+| `c`    | composer                                     | Starts composer                                                                        |
+| `csu`  | composer self-update                         | Updates composer to the latest version                                                 |
+| `cu`   | composer update                              | Updates composer dependencies and `composer.lock` file                                 |
+| `cr`   | composer require                             | Adds new packages to `composer.json`                                                   |
+| `crm`  | composer remove                              | Removes packages from `composer.json`                                                  |
+| `ci`   | composer install                             | Resolves and installs dependencies from `composer.json`                                |
+| `ccp`  | composer create-project                      | Create new project from an existing package                                            |
+| `cdu`  | composer dump-autoload                       | Updates the autoloader                                                                 |
+| `cdo`  | composer dump-autoload --optimize-autoloader | Converts PSR-0/4 autoloading to classmap for a faster autoloader (good for production) |
+| `cgu`  | composer global update                       | Allows update command to run on COMPOSER_HOME directory                                |
+| `cgr`  | composer global require                      | Allows require command to run on COMPOSER_HOME directory                               |
+| `cgrm` | composer global remove                       | Allows remove command to run on COMPOSER_HOME directory                                |
+| `cget` | `curl -s https://getcomposer.org/installer`  | Installs composer in the current directory                                             |