[README] Add link to the security page

It reproduces https://github.com/mui/material-ui#security. My assumption is that there isn't any real reason for having a different tradeoff, so using the same would be better

Signed-off-by: Olivier Tassinari <olivier.tassinari@gmail.com>

README.md

@@ -111,3 +111,7 @@ The [changelog](https://github.com/mui/mui-x/releases) is regularly updated to r
 ## Roadmap
 
 Future plans and high-priority features and enhancements can be found in our [roadmap](https://mui.com/x/introduction/roadmap/).
+
+## Security
+
+For details of supported versions and contact details for reporting security issues, please refer to the [security policy](https://github.com/mui/mui-x/security/policy).