add grafana's k6 load testing tool (#5041)

README.md

@@ -3151,6 +3151,7 @@ _Software written in Go._
 - [Hey](https://github.com/rakyll/hey) - Hey is a tiny program that sends some load to a web application.
 - [httpref](https://github.com/dnnrly/httpref) - httpref is a handy CLI reference for HTTP methods, status codes, headers, and TCP and UDP ports.
 - [jcli](https://github.com/jenkins-zh/jenkins-cli) - Jenkins CLI allows you manage your Jenkins as an easy way.
+- [k6](https://github.com/grafana/k6) - A modern load testing tool, using Go and JavaScript.
 - [kala](https://github.com/ajvb/kala) - Simplistic, modern, and performant job scheduler.
 - [kcli](https://github.com/cswank/kcli) - Command line tool for inspecting kafka topics/partitions/messages.
 - [ko](https://github.com/google/ko) - Command line tool for building and deploying Go applications on Kubernetes