Update README

[PuneetKohli]

• Rearrange README to place "Pre-trained models from SIGGRAPH'18 paper" before "Running the model on a single image pair"
• Add note to indicate that model must be downloaded before running mpi_from_images.py

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[PuneetKohli]

I signed it!

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[PuneetKohli]

Does anyone at Google look at the open PRs for this repo?

[snaves]

Thanks for the suggestion (and apologies for the delay).