Add a docs note about drupal_build_composer not always being optimal [ci skip]

Author: oxyc

docs/deployment/composer.md

@@ -1,5 +1,7 @@
 Drupal VM is configured to use `composer create-project` to build a Drupal 8 codebase by default but supports building Drupal from a custom `composer.json` file as well.
 
+_Note that if you already have a project built using a `composer.json` file you should instead add [Drupal VM as a dependency](composer-dependency.md) of your project. The method described below will make it somewhat difficult to manage your `composer.json` together with Drupal VM as the file will be copied from `drupal_composer_path` into `drupal_composer_install_dir` as a secondary `composer.json`._
+
 1. Copy `example.drupal.composer.json` to `drupal.composer.json` and modify it to your liking.
 2. Use the Composer build system by setting `drupal_build_composer: true` in your `config.yml` (make sure `drupal_build_makefile` and `build_composer_project` are set to `false`).
 3. Configure `drupal_core_path` to point to the webroot directory: `drupal_core_path: {{ drupal_composer_install_dir }}/docroot`