Add recipe for sml-modeline

sml-modeline is a nice minor mode that shows a diagram of the currently displayed buffer area relative to the total buffer size, like a small non-interactive scroll bar located in the modeline.

It's been written by Lennart Borgman as an independent utility within his [nXhtml](http://ourcomments.org/Emacs/nXhtml/doc/nxhtml.html) project.