Suggest using reduced frame rate for Pi Zero

Reducing the framerate to 15 FPS greatly reduces the CPU usage (~50% total) on the Pi Zero W and improves the keyboard response time and helps with drop/repeated keys during my experiments. I'd recommend suggesting this on the wiki somewhere. I'm using the following settings with improved success over the default settings.

```
ustreamer_desired_fps: 15
ustreamer_drop_same_frames: 15
```