Add documentation on metatags

Docs/CalculatedTags.md

@@ -0,0 +1,27 @@
+### \_lat, \_lon
+
+The latitude and longitude of the point (or centerpoint in the case of a way/area)
+
+### \_surface, \_surface:ha
+
+The surface area of the feature, in square meters and in hectare. Not set on points and ways
+
+### \_country
+
+The country code of the property (with latlon2country)
+
+### \_isOpen, \_isOpen:description
+
+If 'opening\_hours' is present, it will add the current state of the feature (being 'yes' or 'no')
+
+### \_width:needed, \_width:needed:no\_pedestrians, \_width:difference
+
+Legacy for a specific project calculating the needed width for safe traffic on a road. Only activated if 'width:carriageway' is present
+
+### \_direction:simplified, \_direction:leftright
+
+\_direction:simplified turns 'camera:direction' and 'direction' into either 0, 45, 90, 135, 180, 225, 270 or 315, whichever is closest. \_direction:leftright is either 'left' or 'right', which is left-looking on the map or 'right-looking' on the map
+
+### \_now:date, \_now:datetime, \_loaded:date, \_loaded:\_datetime
+
+Adds the time that the data got loaded - pretty much the time of downloading from overpass. The format is YYYY-MM-DD hh:mm, aka 'sortable' aka ISO-8601-but-not-entirely

Docs/Making_Your_Own_Theme.md

@@ -80,7 +80,13 @@ Every field is documented in the source code itself - you can find them here:
 - [The `TagRendering`](https://github.com/pietervdvn/MapComplete/blob/master/Customizations/JSON/TagRenderingConfigJson.ts)
 - At last, the exact semantics of tags is documented [here](Docs/Tags_format.md)
 
-## Some pitfalls
+### MetaTags
+
+There are few tags available that are calculated for convenience - e.g. the country an object is located at. [An overview of all these metatags is available here](Docs/CalculatedTags.md)
+
+
+ Some pitfalls
+---------------
 
 ### Not publishing