Add custom repo package resotore

Adds the ability for contributors to pull the Umbraco specific Nuget
packages from the Myget repository using package restore.

https://docs.nuget.org/consume/nuget-config-file

src/NuGet.Config

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <packageSources>
+    <add key="nuget.org" value="https://www.nuget.org/api/v2/" />
+	<add key="umbracocore" value="http://www.myget.org/f/umbracocore/" />
+  </packageSources>
+</configuration>