crossing: Add readme for crossing presets and fields

Author: tordans

data/presets/highway/crossing/README.md

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+# About the crossing presets
+
+## General
+
+These presets use fields `@templates` to make sure all have a similar structure.
+Some presets like `traffic_signals` have additinal fields and use a different field order.
+Try to keep the file structure very similar so they can be easily compared.
+
+## nodes `highway=crossing + crossing=*`
+
+Crossing nodes are handel by `highway/crossing.json` + `highway/crossing/*.json` (as geometry type `vertex`).
+
+Crossing nodes and crossing ways have a big overlap.
+However, there are a few tags that should _only_ be expected on the crossing node. When crossing ways are present, this also means there are separate ways attached with additional nodes that hald the
+
+There are places in OSM that follow a different tagging style where only crossing ways are used (without the vertex nodes). Those regions might have other conventions on where to place
+
+## ways `highway=footway|cycleway|path + *=crossing + crossing=*`
+
+Crossing way presets are duplicated per highway class:
+- `/highway/footway/crossing.json`  + `/highway/footway/crossing/*.json`
+- `/highway/cycleway/crossing.json` + `/highway/cycleway/crossing/*.json`
+- `/highway/path/crossing.json`     + `/highway/path/crossing/*.json`
+
+The crossing presets for `/cycleway*` and `/path*` are considered to be relevant for bike traffic and have additional fields that target bike vs. foot traffic.