Specify how to add the necessary Service Account permissions

[goek-han]

In the docs it is mentioned that owner rights are needed, but it would be nice to also add to link to https://docs.stackit.cloud/stackit/en/assign-permissions-to-a-service-account-134415855.html in

stackit-sdk-go/README.md

Line 102 in 0092424

To authenticate with the SDK, you need a [service account](https://docs.stackit.cloud/stackit/en/service-accounts-134415819.html) with appropriate permissions (e.g., `project.owner`). You can create a service account through the STACKIT Portal.

[bahkauv70]

Thanks for the PR @goek-han! I've just merged it