Update link to supported platforms guide
Update link to supported platforms guide

Fixes #1081.
jskeet/gcloud-dotnet,chrisdunelm/google-cloud-dotnet,chrisdunelm/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet,benwulfe/google-cloud-dotnet,benwulfe/google-cloud-dotnet,evildour/google-cloud-dotnet,googleapis/google-cloud-dotnet,benwulfe/google-cloud-dotnet,evildour/google-cloud-dotnet,chrisdunelm/google-cloud-dotnet,iantalarico/google-cloud-dotnet,googleapis/google-cloud-dotnet,evildour/google-cloud-dotnet,jskeet/google-cloud-dotnet,iantalarico/google-cloud-dotnet,iantalarico/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet,googleapis/google-cloud-dotnet,chrisdunelm/gcloud-dotnet