Add contributor guidelines

[w-is-h]

Politely take them from some other open-source repo (example)

[tomolopolis]

👍 I knew it was hf before clicking 🤣

[w-is-h]

Comments style: https://github.com/huggingface/transformers/tree/master/docs#writing-documentation---specification

[baixiac]

Duplicated with CU-1yfef9z.