document the training API

[zeke]

Followup to #82

[zeke]

Another user ran into this today. @fool1280

[zeke]

training = replicate.trainings.create(
  version="replicate/flan-t5-xl:3ae0799123a1fe11f8c89fd99632f843fc5f7a761630160521c4253149754523",
  input={
    "train_data": "https://storage.googleapis.com/dan-scratch-public/fine-tuning/70k_samples.jsonl",
  },
  destination="replicate/robohacker"
)

module 'replicate' has no attribute 'trainings'