add CI status badge (#1)

README.md

@@ -1,7 +1,8 @@
 # getopt
 [![Go Reference](https://pkg.go.dev/badge/github.com/jon-codes/getopt.svg)](https://pkg.go.dev/github.com/jon-codes/getopt)
-[![codecov](https://codecov.io/github/jon-codes/getopt/graph/badge.svg?token=CF7WDJOFVY)](https://codecov.io/github/jon-codes/getopt)
+[![CI](https://github.com/jon-codes/getopt/actions/workflows/ci.yml/badge.svg)](https://github.com/jon-codes/getopt/actions/workflows/ci.yml)
 [![Go Report Card](https://goreportcard.com/badge/github.com/jon-codes/getopt)](https://goreportcard.com/report/github.com/jon-codes/getopt)
+[![codecov](https://codecov.io/github/jon-codes/getopt/graph/badge.svg?token=CF7WDJOFVY)](https://codecov.io/github/jon-codes/getopt)
 
 Package `getopt` provides a zero-dependency Go implementation of the Unix getopt function for parsing command-line options.