2018 TIGER data conversion scripts, add documentation to /docs/data-sources

data-sources/us-tiger/README.md

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+# US TIGER address data
+
+Convert [TIGER](https://www.census.gov/geo/maps-data/data/tiger.html)/Line dataset of the US Census Bureau to SQL files which can be imported by Nominatim. The created tables in the Nominatim database are separate from OpenStreetMap tables and get queried at search time separately.
+
+The dataset gets updated once per year. Downloading is prown to be slow (can take a full day) and converting them can take hours as well.
+
+Replace '2018' with the current year throughout.
+
+  1. Install the GDAL library and python bindings and the unzip tool
+
+        # Ubuntu:
+        sudo apt-get install python-gdal unzip
+        # CentOS:
+        sudo yum install gdal-python unzip
+
+  2. Get the TIGER 2018 data. You will need the EDGES files
+     (3,233 zip files, 11GB total).
+
+         wget -r ftp://ftp2.census.gov/geo/tiger/TIGER2018/EDGES/
+
+  3. Convert the data into SQL statements. Adjust the file paths in the scripts as needed
+
+        cd data-sources/us-tiger
+        ./convert.sh <input-path> <output-path>
+        
+  4. Maybe: package the created files
+  
+        tar -czf tiger2018-nominatim-preprocessed.tar.gz tiger
+