Update the README.md (Add Status Badge for CI)

[![CI - Build, Test, and Publish SDKs](https://github.com/microsoft/Agent365-python/actions/workflows/ci.yml/badge.svg?branch=main)](https://github.com/microsoft/Agent365-python/actions/workflows/ci.yml)

Update for the CI status badge.