Update NuGet-Config-File.markdown

added nuget extensibility point
machine wide config
added the screenshot
machine wide config updated
Merge branch 'nuget-2.5-deepakv' of https://github.com/NuGet/NuGetDocs into nuget-2.5-deepakv
Merge branch 'master' into nuget-2.5-deepakv
removed c:\temp from the screenshot
Update NuGet-Config-File.markdown

Update the sku info
Merge pull request #105 from NuGet/nuget-2.5-deepakv

Updated doc for NuGet.config extensibility