Add US Census Bureau layers

layers/census-bureau/README.md

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+from the US Census Bureau:
+
+https://www.census.gov/geographies/mapping-files/time-series/geo/cartographic-boundary.html
+
+more description here:
+
+https://www.census.gov/programs-surveys/geography/technical-documentation/naming-convention/cartographic-boundary-file.html
+
+Including the following:
+
+> # File Naming Convention
+> ## 2013 to Present Files
+> 
+> The cartographic boundary files are named cb_yyyy_ss_<entity>_rr.zip where:
+> 
+> * yyyy = 4 digit year
+> * ss = state FIPS code or 'us' for a national level file
+> * entity = the entity name
+> * rr = resolution level
+>    * 500k = 1:500,000
+>    * 5m = 1:5,000,000
+>    * 20m = 1:20,000,000
+
+I prefer the KML options when available since OpenLayers doesn't [citation
+needed?] have built-in support for shapefiles (.shp)? That said, we also do
+transform KML to GeoJSON to be able to edit more easily (and it's sometimes good
+for a smaller file size).
+
+For more information, see e.g.:
+
+https://indicatrix.org/post/shapefiles-in-openlayers/
+
+Census Bureau data appears to be using a different (US-centric, possibly?)
+projection; this may require more work to be accurate going forward.
+
+https://gis.stackexchange.com/a/310949
+
+https://epsg.io/4269