Write 4.x upgrade note (#49)

* Write 4.x upgrade note

* Apply suggestions from code review

Co-authored-by: Benoit Jouhaud <[email protected]>

Co-authored-by: Benoit Jouhaud <[email protected]>

Author: yann-eugone

UPGRADE-4.x.md

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+## Changes made in `EnumInterface`
+
+- `Yokai\EnumBundle\EnumInterface::getChoices` must now return the enum labels as keys of the array.
+- `Yokai\EnumBundle\EnumInterface::getValues` was introduced 
+  and must return all possible enum values.
+- `Yokai\EnumBundle\EnumInterface::getLabel` was introduced 
+  and must return the associated label of any provided enum value.
+
+## Changes made in enum base classes
+
+- `Yokai\EnumBundle\AbstractTranslatedEnum` was removed, 
+  you can now extend `Yokai\EnumBundle\TranslatedEnum` and provide values as constructor argument.
+- `Yokai\EnumBundle\ConfigurableTranslatedEnum` was removed, 
+  you can now extend `Yokai\EnumBundle\TranslatedEnum` instead.
+- `Yokai\EnumBundle\Enum` was introduced, 
+  you can now extend this class and provide choices and names as constructor arguments (recommended).
+
+## Changes made to other classes
+
+- Non enum classes were all made final.
+- `enum_label` Twig function was removed (use `enum_label` Twig filter instead).
+- Most methods now throw exceptions that implements (`Yokai\EnumBundle\Exception\ExceptionInterface`).
+
+## Changes made in documentation
+
+- Due to how popular [service discovering](https://symfony.com/blog/new-in-symfony-3-3-psr-4-based-service-discovery) has become, 
+  doc will only show how to create an enum by creating a new class. 
+  But it is still 100% valid to create new enums without creating a class.