message: add missing step to docs
diff: @@ -33,7 +33,17 @@ We've also removed a lot of the built in helpers that we shipped with
  The migration path is pretty simple, and we've removed some of the pain points
  from writing the `handlebars` templates too. Let's go through what's new and how
  to upgrade.
  +## Add processor
  +An important change is to add the required processor to your `packages/backend/src/plugins/catalog.ts`
  +```diff
  ++import { ScaffolderEntitiesProcessor } from '@backstage/plugin-scaffolder-backend';
  +
  +...
  + const builder = await CatalogBuilder.create(env);
  ++ builder.addProcessor(new ScaffolderEntitiesProcessor());
  + const { processingEngine, router } = await builder.build();
  +```
  ## `backstage.io/v1beta2` -> `scaffolder.backstage.io/v1beta3`
  The most important change is that you'll need to switch over the `apiVersion` in
  @@ -45,6 +55,7 @@ your templates to the new one.
  + apiVersion: scaffolder.backstage.io/v1beta3
  ```
  +
  ## `${{ }}` instead of `"{{ }}"`
  One really big readability issue and cause for confusion was the fact that with
  