Add google colab export link

[abishpius]

It would be nice to add a colab link to the project, so that it is easier for everyone to access. It should be relatively easy to host the fastMRI_tutorial.ipynb in colab.

Thanks, Abish Pius

[mmuckley]

Hello @abishpius, I think Google Colab would be great. The only restriction I can think of is that we're not allowed to redistribute the fastMRI data, and the tutorial notebook uses one of the files from the dataset. I'll ask NYU if they might allow it in this case.