Add CNCF status badge to README (#6534)

* Add CNCF status badge to README

Highlighting PipeCD's status in the cloud-native ecosystem

Signed-off-by: Ayush More <ayushmore42595@gmail.com>

* Update README.md

Signed-off-by: Ayush More <ayushmore42595@gmail.com>

---------

Signed-off-by: Ayush More <ayushmore42595@gmail.com>

Author: Ayushmore1214

README.md

@@ -8,6 +8,7 @@
 [![Documentation](https://img.shields.io/badge/Documentation-pipecd-informational.svg)](https://pipecd.dev/docs/)
 [![Slack](https://img.shields.io/badge/Slack-%23pipecd-informational.svg)](https://app.slack.com/client/T08PSQ7BQ/C01B27F9T0X)
 [![Twitter](https://img.shields.io/twitter/url/https/twitter.com/pipecd_dev.svg?style=social&label=Follow%20%40pipecd_dev)](https://twitter.com/pipecd_dev)
+[![CNCF Status](https://img.shields.io/badge/CNCF%20Status-Sandbox-lightgrey.svg)](https://www.cncf.io/projects/pipecd/)
 
 <p align="center">
   <img src="https://github.com/pipe-cd/pipecd/blob/master/docs/static/images/logo.png" width="180"/>