composer: add page (#1329)

pages/common/composer.md

@@ -0,0 +1,23 @@
+# composer
+
+> A package-based dependency manager for PHP projects.
+
+- Add a package as a dependency for this project, adding it to `composer.json`:
+
+`composer require {{user/package-name}}`
+
+- Install all the dependencies in this project's `composer.json`:
+
+`composer install`
+
+- Uninstall a package from this project, removing it as a dependency from `composer.json`:
+
+`composer remove {{user/package-name}}`
+
+- Update all the dependencies in this project's `composer.json`:
+
+`composer update`
+
+- Update composer to the latest version:
+
+`composer self-update`