Add NuGet official to config project

Signed-off-by: José Simões <jose.simoes@eclo.solutions>

Author: josesimoes

source/NuGet.Config

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <configuration>
     <packageSources>
+        <add key="NuGet" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
         <add key="MyGet nanoFramework dev" value="https://www.myget.org/F/nanoframework-dev/api/v3/index.json" protocolVersion="3" />
     </packageSources>
 </configuration>