add ci badge to readme

Author: nfriedly

README.md

@@ -1,5 +1,7 @@
 # ratelimit-header-parser
 
+[![CI](https://github.com/express-rate-limit/ratelimit-header-parser/actions/workflows/ci.yml/badge.svg)](https://github.com/express-rate-limit/ratelimit-header-parser/actions/workflows/ci.yml)
+
 Parse RateLimit headers of various forms, including the combined form from
 [draft 7](https://datatracker.ietf.org/doc/html/draft-ietf-httpapi-ratelimit-headers-07)
 of the