[docs] Link the OpenSSF Best Practices card (#34331)

README.md

@@ -30,6 +30,7 @@
 [![Average time to resolve an issue](https://isitmaintained.com/badge/resolution/mui/material-ui.svg)](https://isitmaintained.com/project/mui/material-ui 'Average time to resolve an issue')
 [![Crowdin](https://badges.crowdin.net/material-ui-docs/localized.svg)](https://translate.mui.com/project/material-ui-docs)
 [![Open Collective backers and sponsors](https://img.shields.io/opencollective/all/mui)](https://opencollective.com/mui)
+[![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/1320/badge)](https://bestpractices.coreinfrastructure.org/projects/1320)
 
 </div>