docs: update v11 link in README.md file (#989)

Co-authored-by: Michael Prentice <[email protected]>

Author: 2 people

material.angular.io/material.angular.io/material.angular.io/material.angular.io/material.angular.io/README.md

@@ -9,7 +9,8 @@ Versions of this site are also available for
 - [v8](https://v8.material.angular.io/)
 - [v9](https://v9.material.angular.io/)
 - [v10](https://v10.material.angular.io/)
-- [v11](https://material.angular.io/)
+- [v11](https://v11.material.angular.io/)
+- [v12](https://material.angular.io/)
 
 ## Contributing