[Commands] In-detail instructions to create dummy data folder
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awesome !
### Dummy data command 

This PR adds a new command `python nlp-cli dummy_data <path_to_dataset_folder>` that gives in-detail instructions on how to add the dummy data files. 

It would be great if you can try it out by moving the current dummy_data folder of any dataset in `./datasets` with `mv datasets/<dataset_script>/dummy_data datasets/<dataset_name>/dummy_data_copy` and running the command `python nlp-cli dummy_data ./datasets/<dataset_name>` to see if you like the instructions. 

### CONTRIBUTING.md
Also the CONTRIBUTING.md is made cleaner including a new section on "How to add a dataset". 

### Current PRs 
It would be nice if we can try out if this command helps current PRs, *e.g.* #169  to add a dataset. I comment on those PRs.
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