Add note about @anotherjames article

Author: cweagans

README.md

@@ -180,6 +180,19 @@ If you want to avoid this behavior, add a specific key to the `extra` section:
 
 Or provide an environment variable `COMPOSER_PATCHES_SKIP_REPORTING` with a config.
 
+## Patching composer.json in dependencies
+
+This doesn't work like you'd want. By the time you're running `composer install`,
+the metadata from your dependencies' composer.json has already been aggregated by
+packagist (or whatever metadata repo you're using). Unfortunately, this means that
+you cannot e.g. patch a dependency to be compatible with an earlier version of PHP
+or change the framework version that a plugin depends on.
+
+@anotherjames over at @computerminds wrote an article about how to work around
+that particular problem for a Drupal 8 -> Drupal 9 upgrade:
+
+[Apply Drupal 9 compatibility patches with Composer](https://www.computerminds.co.uk/articles/apply-drupal-9-compatibility-patches-composer) ([archive](https://web.archive.org/web/20210124171010/https://www.computerminds.co.uk/articles/apply-drupal-9-compatibility-patches-composer))
+
 ## Difference between this and netresearch/composer-patches-plugin
 
 - This plugin is much more simple to use and maintain