Modification of README.md file to incorporate 2d_diffusion_autoencoder info.

[virginiafdez]

In the previous (closed) PR (#1871) we had to do a hard reset and the README.md modifications were not incorporated at the end.
This is just an addition of the notebook info to the generation README file.

Checks

• Avoid including large-size files in the PR.
• Clean up long text outputs from code cells in the notebook.
• For security purposes, please check the contents and remove any sensitive info such as user names and private key.
• Ensure (1) hyperlinks and markdown anchors are working (2) use relative paths for tutorial repo files (3) put figure and graphs in the ./figure folder
• Notebook runs automatically ./runner.sh -t <path to .ipynb file>