add T5 readme

README.md

convlab/base_models/t5/README.md

convlab/base_models/t5/dst/__init__.py

convlab/base_models/t5/dst/dst.py

convlab/base_models/t5/key2gen/create_data.py

convlab/base_models/t5/key2gen/dataset_godel.py

convlab/base_models/t5/key2gen/dataset_vanilla.py

convlab/base_models/t5/key2gen/eval.ipynb

convlab/base_models/t5/key2gen/evaluate.py

convlab/base_models/t5/key2gen/features.py

convlab/base_models/t5/key2gen/finetune.sh

convlab/base_models/t5/key2gen/metric.py

convlab/base_models/t5/nlg/__init__.py

convlab/base_models/t5/nlg/nlg.py

convlab/base_models/t5/nlu/__init__.py

convlab/base_models/t5/nlu/nlu.py