docs: add circleci status to README.md

Author: skyllo

README.md

@@ -1,5 +1,7 @@
 # FetchEventSource
 
+[![FetchEventSource](https://circleci.com/gh/skyllo/fetch-event-source.svg?style=svg)](https://circleci.com/gh/skyllo/fetch-event-source)
+
 Fetch implementation of EventSource with more functionality like setting HTTP headers and support for Node.js. Written in TypeScript with the types includes in the package.
 
 # Features