add gitter link (#8539)

* missing 'alt' attribute added

* missing 'alt' attribute added

* replaced 'new URL' to 'new URI' per Java 11 target codebase

* Revert "replaced 'new URL' to 'new URI' per Java 11 target codebase"

This reverts commit aeef4c1.

* add jenkins gitter chat link at README.md

README.md

@@ -8,6 +8,7 @@
 [![Jenkins LTS Release](https://img.shields.io/endpoint?url=https%3A%2F%2Fwww.jenkins.io%2Fchangelog-stable%2Fbadge.json)](https://www.jenkins.io/changelog-stable)
 [![Docker Pulls](https://img.shields.io/docker/pulls/jenkins/jenkins.svg)](https://hub.docker.com/r/jenkins/jenkins/)
 [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/3538/badge)](https://bestpractices.coreinfrastructure.org/projects/3538)
+[![Gitter](https://img.shields.io/gitter/room/jenkinsci/jenkins)](https://app.gitter.im/#/room/#jenkinsci_jenkins:gitter.im)
 
 In a nutshell, Jenkins is the leading open-source automation server.
 Built with Java, it provides over 1,800 [plugins](https://plugins.jenkins.io/) to support automating virtually anything,