Update README.md to add slack badge and announcement (#2653)

* Update README.md

Add slack workspace announcement at top of README for a couple weeks.

* Update README.md

README.md

@@ -1,5 +1,10 @@
 [![CI](https://github.com/opensearch-project/security/workflows/CI/badge.svg?branch=main)](https://github.com/opensearch-project/security/actions) [![](https://img.shields.io/github/issues/opensearch-project/security/untriaged?labelColor=red)](https://github.com/opensearch-project/security/issues?q=is%3Aissue+is%3Aopen+label%3A"untriaged") [![](https://img.shields.io/github/issues/opensearch-project/security/security%20vulnerability?labelColor=red)](https://github.com/opensearch-project/security/issues?q=is%3Aissue+is%3Aopen+label%3A"security%20vulnerability") [![](https://img.shields.io/github/issues/opensearch-project/security)](https://github.com/opensearch-project/security/issues) [![](https://img.shields.io/github/issues-pr/opensearch-project/security)](https://github.com/opensearch-project/security/pulls) 
 [![](https://img.shields.io/codecov/c/gh/opensearch-project/security)](https://app.codecov.io/gh/opensearch-project/security) [![](https://img.shields.io/github/issues/opensearch-project/security/v2.4.0)](https://github.com/opensearch-project/security/issues?q=is%3Aissue+is%3Aopen+label%3A"v2.4.0") [![](https://img.shields.io/github/issues/opensearch-project/security/v3.0.0)](https://github.com/opensearch-project/security/issues?q=is%3Aissue+is%3Aopen+label%3A"v3.0.0")
+[![Slack](https://img.shields.io/badge/Slack-4A154B?&logo=slack&logoColor=white)](https://opensearch.slack.com/archives/C051Y637FKK)
+
+
+
+## Announcement: The Slack workspace is live! Please join the [conversation](https://opensearch.slack.com/archives/C051Y637FKK). 
 
 <img src="https://opensearch.org/assets/img/opensearch-logo-themed.svg" height="64px">