Update release processes to talk about prepare-release.sh
Update release processes to talk about prepare-release.sh

googleapis/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet,googleapis/google-cloud-dotnet,googleapis/google-cloud-dotnet,jskeet/gcloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet