Tagging error when sidewalk:*=separate, but not sidewalk present

In the list of tagging errors in the `README.md` an important tagging error is missing: A street is tagged with `sidewalk:both=separate` (or something similar), but there is no separate sidewalk mapped. This typically causes routers to make detours, because the can't find a way along this street.

Some [examples](https://www.openstreetmap.org/#map=17/50.813077/7.176105) can be found in Troisdorf. (I added notes there.)