Add NuGet.config

src/dotnet/NuGet.config

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+
+  <packageRestore>
+    <!-- Allow NuGet to download missing packages -->
+    <add key="enabled" value="True" />
+    <!-- Automatically check for missing packages during build -->
+    <add key="automatic" value="True" />
+  </packageRestore>
+
+  <packageSources>
+    <add key="NuGet" value="https://api.nuget.org/v3/index.json" />
+  </packageSources>
+  
+</configuration>