feature: add service groups to readme

Author: Your Name

README.md

@@ -83,6 +83,7 @@ Below you can find a list of the STACKIT services already available in the CLI (
 | Redis                              | `redis`                                                                                                              | :white_check_mark:        |
 | Resource Manager                   | `project`                                                                                                            | :white_check_mark:        |
 | Secrets Manager                    | `secrets-manager`                                                                                                    | :white_check_mark:        |
+| Security Groups                     | `beta security-group`                                                                                                 | :white_check_mark: (beta) |
 | Server Backup Management           | `beta server backup`                                                                                                 | :white_check_mark: (beta) |
 | Server Command (Run Command)       | `beta server command`                                                                                                | :white_check_mark: (beta) |
 | Service Account                    | `service-account`                                                                                                    | :white_check_mark:        |