chore: add fern badge to readme (#185)

Co-authored-by: azul <[email protected]>

README.md

@@ -12,6 +12,7 @@
 [![Downloads](https://pepy.tech/badge/nixtla)](https://pepy.tech/project/nixtla)
 [![Downloads](https://pepy.tech/badge/nixtla/month)](https://pepy.tech/project/nixtla)
 [![Downloads](https://pepy.tech/badge/nixtla/week)](https://pepy.tech/project/nixtla)
+[![fern shield](https://img.shields.io/badge/%F0%9F%8C%BF-SDK%20generated%20by%20Fern-brightgreen)](https://buildwithfern.com/?utm_source=nixtla/nixtla/readme)
 
 **TimeGPT** is a production ready, generative pretrained transformer for time series. It's capable of accurately predicting various domains such as retail, electricity, finance, and IoT with just a few lines of code 🚀. </div>