Add CI badge to README

Author: msabramo

README.md

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 # Adobe Firefly Python Client
 
+[![CI](https://github.com/msabramo/python-firefly/actions/workflows/ci.yml/badge.svg?branch=main)](https://github.com/msabramo/python-firefly/actions/workflows/ci.yml)
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 A Python client for the Adobe Firefly API,
 created using the [Adobe Firefly API Documentation].