added a basic RSS feed on the package detail site for issue 541

I added a very basic feed to the package detail site. It uses the SyndicationItem/SyndicationFeed APIs, but doesn't use the existing NuGet OData Feed System. With this the "public feed syndication" feature would be independent to any API changes. 

RSS might be a bit "old school", but there are some pretty cool scenarios possible in combination with IFTTT etc.

Related issue: https://github.com/NuGet/NuGetGallery/issues/541