docs: add Open VSX badge to main README

Author: manojmallick

README.md

@@ -26,6 +26,7 @@
 [![Changelog](https://img.shields.io/badge/changelog-CHANGELOG.md-blue)](CHANGELOG.md)
 [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](CONTRIBUTING.md)
 [![VS Code](https://img.shields.io/badge/VS%20Code-extension-0078d4?logo=visual-studio-code)](https://marketplace.visualstudio.com/items?itemName=manojmallick.sigmap)
+[![Open VSX](https://img.shields.io/open-vsx/v/manojmallick/sigmap?color=a251e3&label=Open%20VSX&logo=vscodium)](https://open-vsx.org/extension/manojmallick/sigmap)
 
 </div>