Add tip for generator authoring

docs/generators.md

@@ -1,4 +1,9 @@
 # Overview
+
+!!! tip
+
+    Drush 11 and prior required generators to define a [drush.services.yml file](https://www.drush.org/11.x/dependency-injection/#services-files). This is no longer used with Drush 12+ generators. See [create() method](dependency-injection.md#create-method) for injecting dependencies.
+
 Generators jump start your coding by building all the boring boilerplate code for you. After running the [generate command](commands/generate.md), you have a guide for where to insert your custom logic.
 
 Drush's generators reuse classes provided by the excellent [Drupal Code Generator](https://github.com/Chi-teck/drupal-code-generator) project. See its [Commands directory](https://github.com/Chi-teck/drupal-code-generator/tree/3.x/src/Command) for inspiration.