Modification of README.md file to incorporate 2d_diffusion_autoencoder info. (#1876)

In the previous (closed) PR
(https://github.com/Project-MONAI/tutorials/pull/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
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Avoid including large-size files in the PR.
- [x] Clean up long text outputs from code cells in the notebook.
- [x] For security purposes, please check the contents and remove any
sensitive info such as user names and private key.
- [x] 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
- [x] Notebook runs automatically `./runner.sh -t <path to .ipynb file>`

---------

Signed-off-by: Virginia <virginia.fernandez@kcl.ac.uk>
Co-authored-by: Virginia <virginia.fernandez@kcl.ac.uk>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>