Add README

README.md

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+*The terminal commands listed in these instructions are specific to Unix shells such as those found
+on Linux and macOS and might need to be adapted to run on other systems.*
+
+Install Python and the dependencies listed in `requirements.txt`: `pip3 install --user -r requirements.txt`
+
+Fetch data from Overpass with this query:
+
+```overpass-ql
+[out:json][timeout:25];
+(
+  nwr["wikidata"]({{bbox}});
+  nwr[~":wikidata$"~".*"]({{bbox}});
+);
+out tags;
+```
+
+Save the raw OSM data as `export.json` in this directory.
+
+Open a terminal in this directory and run `python3 osm_wikidata_check.py < export.json`.