Increment version and add release notes for TFF 0.6.0.

README.md

RELEASE.md

docs/tutorials/custom_federated_algorithms_1.ipynb

docs/tutorials/custom_federated_algorithms_2.ipynb

...torials/federated_learning_for_image_classification.ipynb

docs/tutorials/federated_learning_for_text_generation.ipynb

tensorflow_federated/tools/setup.py