Add recommendation for PC wheels

[aamott]

Didn't know where to put this, so sorry for that, but it made such a difference with my belted z setup that I had to share.

After something like two years of using this mod, I decided to try PC wheels on the z axis. I'm on the old a4988 drivers, but it looks better than any printer I've ever used before. It's an insane difference switching between PC and Delrin wheels. $3 for the whole upgrade (used the old bearings) but it looks like the work of linear rails. I'd like to recommend a note on PC wheels in the readme because of this.