Briefly document the differences between Masakhane-MTs 4 notebooks

[jaderabbit]

Currently Masakhane MT has the following 4 notebooks:

1. The primary starter notebook: https://github.com/masakhane-io/masakhane-mt/blob/master/starter_notebook.ipynb
2. The notebook with custom data: https://github.com/masakhane-io/masakhane-mt/blob/master/starter_notebook-custom-data.ipynb
3. Not sure what this does, but assumedly it handles google drive better? https://github.com/masakhane-io/masakhane-mt/blob/master/starter_notebook_gdrive.ipynb
4. Retrain from check point 🤷 : https://github.com/masakhane-io/masakhane-mt/blob/master/starter_notebook_retrain_from_checkpoint.ipynb

Currently, I'm not sure where the differences are, and under what scenarios to use them .

Definition of done:

A table/bullet point for each notebook, on the main Masakhane README, explaining what the differences are and in which scenario to use them.