adds rss feed <link> to template <head>

First, thank you very much for Haskell News. It is the only source of info I use to stay updated on all things Haskell.

I was trying to find the feed URL and ended up going to the source code to dig it out. This pull request just throws it into a `<link>` tag in the `<head>` of the site template. Maybe it'll make it easier for others in the future to find the feed?

Thanks again!