Added nightly nuget feed

Author: bergmania

NuGet.config

@@ -3,6 +3,7 @@
   <packageSources>
     <add key="nuget.org" value="https://www.nuget.org/api/v2/" />
     <add key="Umbraco Prereleases" value="https://www.myget.org/F/umbracoprereleases/api/v3/index.json" />
+    <add key="Umbraco Nightly" value="https://www.myget.org/F/umbraconightly/api/v3/index.json" />
     <add key="UmbracoStorageProviders@Local" value="https://pkgs.dev.azure.com/umbraco/Umbraco.StorageProviders/_packaging/UmbracoStorageProviders%40Local/nuget/v3/index.json" />
   </packageSources>
 </configuration>