Add readme in distributed_training

[ ] 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>

Project-MONAI / tutorials