Update CONTRIBUTING.md

Added Airbnb example

CONTRIBUTING.md

@@ -25,3 +25,4 @@ We have assigned priorities to the items below. See priority guidance:
 | TensorFlow serving end-to-end | How to perform TensorFlow serving on Kubeflow e2e | **P0** | TODO | TensorFlow | TODO | TODO | TODO |
 | [Zillow housing prediction](https://www.kaggle.com/c/zillow-prize-1/kernels) | Zillow's home value prediction on Kaggle | **P0** | High prize Kaggle competition w/ opportunity to show XGBoost | XGBoost | puneith | Google | TODO |
 | [Mercari price suggestion challenge](https://www.kaggle.com/c/mercari-price-suggestion-challenge) | Automatically suggest product proces to online sellers | **P0** | | | | | |
+| Airbnb new user bookings | Where will a new guest book their first travel experience | | | | | | |