Add CLI to the developer tools listed in README (#13)

Author: joaopalet

README.md

@@ -1,12 +1,17 @@
 ## Overview
+
 This repository is the source for STACKIT REST API specifications in the [OpenAPI Specification](https://github.com/OAI/OpenAPI-Specification) format (OAS), which are used for the STACKIT Developer Tools:
+
 - [STACKIT SDK for Go](https://github.com/stackitcloud/stackit-sdk-go)
 - [STACKIT Terraform Provider](https://github.com/stackitcloud/terraform-provider-stackit)
+- [STACKIT CLI](https://github.com/stackitcloud/stackit-cli)
 
 This source is filled in an automated way only.
 
 ## Contribute and Report Issues
+
 No contributions and no issue reporting are accepted for this repository. Both can be done in the internal api-specification repository.
 
 ## License
+
 Apache 2.0