TUTORIAL: Add {poly,taffy,…}bar example

[slotThe]

Problem Description

Since a lot of people use xmobar as a status bar for xmonad, the tutorial naturally focuses on that use-case. However, polybar seems to have become quite a popular choice by now. Since we want to deemphasise pipe-based workflows, and polybar does not support reading from an arbitrary property, the easiest ways to stick the two together seem to be through polybar's EMWH functionality, or via dbus. It would be nice if the tutorial had a section that talks about alternative status bars like polybar and how to get these working.