Add custom javascript snippets to calculate tags

Docs/CalculatingExtraTags.md

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+# Extra, automatically created tags
+
+In some cases, it is useful to have some tags calculated based on other properties.
+
+Some useful tags are available by default (e.g. `_lat`, `_lon`, `_country`) and are always available (have a lookt at [CalculatedTags.md](CalculatedTags.md) to see an overview).
+
+It is also possible to calculate your own tags - but this requires some javascript knowledge. 
+
+Before proceeding, some warnings:
+
+- **DO NOT DO THIS AS BEGINNER**
+- **Only do this if all other techniques fail**. This should _not_ be done to create a rendering effect, only to calculate a specific vaue
+- **THIS MIGHT BE DISABLED WITHOUT ANY NOTICE ON UNOFFICIAL THEMES**. As unofficial themes might be loaded from the internet, this is the equivalent of injecting arbitrary code into the client. It'll be disabled if abuse occurs.
+
+In the layer object, add a field `calculatedTags`, e.g.:
+
+```
+  "calculatedTags": {
+    "_someKey": "javascript-expression",
+    "name": "tags.name ?? tags.ref ?? tags.operator"
+  }
+```