add CI badge

Author: eli-darkly

README.md

@@ -2,6 +2,7 @@
 
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)
 [![GoDoc](https://godoc.org/github.com/launchdarkly/go-ntlm-proxy-auth?status.svg)](https://godoc.org/github.com/launchdarkly/go-ntlm-proxy-auth)
+[![Circle CI](https://circleci.com/gh/launchdarkly/go-ntlm-proxy-auth.svg?style=svg)](https://circleci.com/gh/launchdarkly/go-ntlm-proxy-auth)
 
 With this package, you can connect to http/https servers protected by an NTLM proxy in Golang.