Add BORDERS_CLEANUP param to .env (#886)

This should be a noop, but will help users that it is available.

.env

@@ -18,4 +18,9 @@ BBOX=-180.0,-85.0511,180.0,85.0511
 MIN_ZOOM=0
 MAX_ZOOM=14
 
+# Hide some output from Mapnik tile generation for clarity
 FILTER_MAPNIK_OUTPUT=1
+
+# Some area data like openstreetmap.fr can contain invalid references
+# that must be cleaned up before using it for borders -- set it to true.
+BORDERS_CLEANUP=false