Added info about and link to Unity GDK#9

ElleEss merged 2 commits intomasterspatialos/sdk-for-unity-starter-project:masterfrom

docs/CON-3273-point-to-UnityGDKspatialos/sdk-for-unity-starter-project:docs/CON-3273-point-to-UnityGDKCopy head branch name to clipboard