Update README with more informations

[chaithyagr]

The cover image can be update to add:

• Update trajectory with gif from the example, show both 2D and 3D trajectories.
• Add density stuff in image
• Explain autodiff to learn trajectory and learn reconstructors.
• Add the benchmark and link to it.

[paquiteau]

I don't think the gif should be on the cover image (then the whole cover image needs to be a gif), but they can definitely go in the readme as well. I think its more about providing the right information in the README and lure people in to reading the doc (and the paper when its accepted)