Add Tutorial With Steps to Migrate from Jekyll.

As promised in https://github.com/spf13/hugo/pull/226, here's a quick tutorial that explains the rough outline of what's involved to migrate from `Jekyll` to `Hugo`. I go in more depth on some details that were particularly relevant to my case but maybe the community can build on it and add more details as appropriate. 

I created a new section called `tutorials` since this seemed like the natural way to go.