Tutorials for developers

[dyxohjl666]

Add raw tutorials to incorporate a new task

[qolina]

We should move the documentation outside of the repo, to keep README.md simple and clean. One way is to consider websites such as readthedocs like TextAttack (https://textattack.readthedocs.io/en/latest/). The second way is to use huggingface hub, like Transformers (https://huggingface.co/docs/transformers/index). The advantage of the latter is we will also provide demo page in huggingface.