docs(fl-operator): document how to add new apis inside the fl-operator

components/fl-operator/README.md

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+# FL Operator
+
+Kubernetes operator in charge of scheduling federated learning clients (e.g flower-clients) on-demand based on running
+fl-suite workflows.
+
+## Development
+
+This project follows the [operator-sdk project layout](https://sdk.operatorframework.io/docs/overview/project-layout/#operator-sdk-project-layout).
+
+### Create a new API
+
+The `operator-sdk` is used to generate new APIs. For installation instructions check the [documentation](https://sdk.operatorframework.io/docs/installation/).
+
+To create a new API with resources and controllers use the following command:
+
+```shell
+# Replace $KIND_NAME with the new API name
+operator-sdk create api --verbose --group fl --version v1alpha1 --kind $KIND_NAME --resource --controller
+```