Add MT5ForConditionalGeneration as supported arch. to summarization README
see #11960
Thanks a lot for fixing this!

Could you also update [translation readme](https://github.com/huggingface/transformers/blob/master/examples/pytorch/translation/README.md) ?<|||||>> Could you also update [translation readme](https://github.com/huggingface/transformers/blob/master/examples/pytorch/translation/README.md) ?

Done.