Re-added dotnet-core feed

Author: ncave

src/fsharp/FSharp.Compiler.Service.ProjectCracker.netcore/NuGet.Config

@@ -4,6 +4,6 @@
     <add key="fsharp-daily" value="https://www.myget.org/F/fsharp-daily/api/v3/index.json" />
     <!-- MSBuild -->
     <add key="dotnet-buildtools" value="https://dotnet.myget.org/F/dotnet-buildtools/api/v3/index.json" />
-
+    <add key="dotnet-core" value="https://dotnet.myget.org/F/dotnet-core/api/v3/index.json" />
   </packageSources>
 </configuration>