docs: Add status badge (#19)

Author: keelerm84

.github/workflows/ci.yml

@@ -1,4 +1,4 @@
-name: Quality control checks
+name: Run CI
 
 on:
   push:

README.md

@@ -1,6 +1,6 @@
 # LaunchDarkly SSE Client for Python
 
-[![Circle CI](https://img.shields.io/circleci/project/launchdarkly/python-eventsource.png)](https://circleci.com/gh/launchdarkly/python-eventsource)
+[![Run CI](https://github.com/launchdarkly/python-eventsource/actions/workflows/ci.yml/badge.svg)](https://github.com/launchdarkly/python-eventsource/actions/workflows/ci.yml)
 [![PyPI](https://img.shields.io/pypi/v/launchdarkly-eventsource.svg?maxAge=2592000)](https://pypi.python.org/pypi/launchdarkly-eventsource)
 [![Documentation](https://img.shields.io/static/v1?label=GitHub+Pages&message=API+reference&color=00add8)](https://launchdarkly.github.io/python-eventsource)