Merge pull request #1 from xamarin/net-5-preview-8

Add dotnet-tools package source

Author: KirillOsenkov

NuGet.config

@@ -27,6 +27,7 @@
     <add key="aspnet-entityframeworkcore" value="https://dotnetfeed.blob.core.windows.net/aspnet-entityframeworkcore/index.json" />
     <add key="aspnet-extensions" value="https://dotnetfeed.blob.core.windows.net/aspnet-extensions/index.json" />
     <add key="gRPC repository" value="https://grpc.jfrog.io/grpc/api/nuget/v3/grpc-nuget-dev" />
+    <add key="dotnet-tools" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-tools/nuget/v3/index.json" />
   </packageSources>
   <disabledPackageSources>
     <clear />