chore(doc): add upgrade doc (#120)

docs/upgrading-9.0.md

@@ -1,4 +1,28 @@
 # Upgrading from 8.x to 9.0
 
-We are still finalizing the 9.0 release features. In the meantime, you can check the
-[9.0 changelog](https://github.com/jolicode/automapper/blob/9.0/CHANGELOG.md)
+9.0 is major release of AutoMapper. It brings a lot of new features and improvements. We recommend first [to check 
+the new documentation](./index.md) to see if the new features are useful for your project.
+
+If you upgrade from 8.x to 9.0, you will need to make some changes to your code, but most of existing behavior should
+still work.
+
+## Bundle
+
+If you use the bundle, it is now integrated in the main package. You can remove the `jolicode/automapper-bundle` package from your
+`composer.json` file.
+
+Then you have to use the new namespace for the bundle:
+
+```php
+use AutoMapper\Symfony\Bundle\AutoMapperBundle;
+```
+
+You will also need to update the bundle configuration, see the [bundle documentation](./bundle/configuration.md) for more
+information.
+
+## Custom Transformers
+
+The `CustomPropertyTransformerInterface` and `CustomModelTransformerInterface` have been removed in favor of the 
+`PropertyTransformerInterface` interface handling both case.
+
+See the [transformers documentation](./mapping/transformer.md#creating-a-custom-transformer) for more information.