Update README.md

README.md

@@ -27,6 +27,9 @@ spatial_ref_sys |     0.02|        0|
 
 ![tables schema](FACT_bldgs.png "Tables")
 
+Properties in footprints contain a dictionary with height (-1 if missing) and confidence, example:
+>{ "height": -1.0, "confidence": -1.0 }
+
 **All spatial objects are converted to EPSG:4326.**
 
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