Batch size preferences

[mattarderne]

Would be helpful to know what are valid batch_size_preferences fields? Wasn't able to find this in the docs, and from the error log determined that {'full_table_streams': []} is a valid entry, but in any case, the dictionary just cannot be empty.

eg: { a : "a" } also works.

Suggest changing the README to

{
  "client_id" : 1234,
  "token" : "asdkjqbawsdciobasdpkjnqweobdclakjsdbcakbdsac",
  "small_batch_url": "https://api.stitchdata.com/v2/import/batch",
  "big_batch_url": "https://api.stitchdata.com/v2/import/batch",
  "batch_size_preferences": { a : "a" }
}