Include instructions to docker login to access GHCR OCI repository

[whatnick]

Describe the bug
OCI repositories require login, this should be made as note in the Retina install instructions.

To Reproduce
Run the supplied helm install command in docs.

helm upgrade --install retina oci://ghcr.io/microsoft/retina/charts/retina \
    --version $VERSION \
    --namespace kube-system \
    --set image.tag=$VERSION \
    --set operator.tag=$VERSION \
    --set image.pullPolicy=Always \
    --set logLevel=info \
    --set os.windows=true \
    --set operator.enabled=true \
    --set operator.enableRetinaEndpoint=true \
    --skip-crds \
    --set enabledPlugin_linux="\[dropreason\,packetforward\,linuxutil\,dns\,packetparser\]" \
    --set enablePodLevel=true \
    --set remoteContext=true
Release "retina" does not exist. Installing it now.
Error: error getting credentials - err: exec: "docker-credential-desktop.exe": executable file not found in $PATH, out: ``

Expected behavior
Install has OCI caveats.