Add alternative to custom installers (#10149)

Taken from composer/installers readme

doc/articles/custom-installers.md

@@ -13,6 +13,16 @@ library.
 In these cases you could consider creating a Custom Installer to handle your
 specific logic.
 
+## Alternative to custom installers with Composer 2.1+
+
+As of Composer 2.1, the `Composer\InstalledVersions` class has a
+[`getInstalledPackagesByType`](https://getcomposer.org/doc/07-runtime.md#knowing-which-packages-of-a-given-type-are-installed)
+method which can let you figure out at runtime which plugins/modules/extensions are installed.
+
+It is highly recommended to use that instead of building new custom
+installers if you are building a new application. This has the advantage of leaving
+all vendor code in the vendor directory, and not requiring custom installer code.
+
 ## Calling a Custom Installer
 
 Suppose that your project already has a Custom Installer for specific modules