Documentation how to write categories in YAML

doc/CategoryAsYAML.md

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+Categories can be created as YAML files. This is much simpler as JSON files and allows adding comments.
+
+A simple example:
+
+```yaml
+type: overpass
+# From zoom level 15 on, load all node, ways and relations with amenity=restaurant.
+query:
+  15: nwr[amenity=restaurant]
+```
+
+Another example, showing fountains from z15 and (additionally) drinking_water from z17:
+```yaml
+type: overpass
+query:
+  15: nwr[amenity=fountain]
+  17: |
+    (
+      nwr[amenity=fountain];
+      nwr[amenity=drinking_water];
+    )
+feature:
+  description: |
+    {{ tagTrans('amenity', tags.amenity) }}
+  markerSign: |
+    {% if tags.amenity == 'fountain' %}
+    ⛲
+    {% elseif tags.amenity == 'drinking_water' %}
+    🚰
+    {% endif %}
+```
+
+Roads, with different color depending on its priority:
+```
+type: overpass
+name: 
+  en: Roads # English name of the category
+query:
+  9: way[highway~"^(motorway|trunk)$"];
+  11: way[highway~"^(motorway|trunk|primary)$"];
+  13: way[highway~"^(motorway|trunk|primary|secondary|tertiary)$"];
+  15: way[highway~"^(motorway|trunk|primary|secondary|tertiary|road|residential)$"];
+feature:
+  description: |
+    {{ tagTrans('highway', tags.highway) }}
+  markerSymbol: # empty, to hide the marker
+  listMarkerSymbol: line
+  style:
+    width: 4
+    color: |
+      {% if tags.highway == 'motorway' %}#ff0000
+      {% elseif tags.highway == 'trunk' %}#ff3f00
+      {% elseif tags.highway == 'primary' %}#ff7f00
+      {% else %}#ffff00{% endif %}
+```