More useful instructions should be added into `add_new_datasets_into_sdk.md`

More useful instructions could be added into [`add_new_datasets_into_sdk.md`](https://github.com/ExpressAI/DataLab/blob/main/docs/SDK/add_new_datasets_into_sdk.md)

* templates of data loader scripts for different tasks (with the help of [`dataset_info.jsonl`](https://github.com/ExpressAI/DataLab/blob/main/utils/dataset_info.jsonl)
* when and how to add a new task schema