[OSM-dev] Expanded osm2pgsql multi-backend usage example

[Paul Norman]

A fairly new feature in osm2pgsql is the multi backend, a backend that 
allows multiple custom tables. Recently committed is an example multi 
style which creates two tables, one for buildings and another for bus 
stops. Although not too useful in itself, this serves t o show how 
osm2pgsql can be used with Lua transforms and the multi backend to 
import data in more complicated schemas and match more other schemas, 
particularly traditional GIS schemas.

This is somewhat of a feature overlap with imposm2/3 which also allows 
for arbitrary table schemas, but there are other differences in the 
feature sets.

https://github.com/openstreetmap/osm2pgsql/blob/master/docs/multi.md has 
the multi documentation, and the files multi.lua and multi.style.json 
are the example of what can be done.

Feedback is always welcome, and issues can be reported at 
https://github.com/openstreetmap/osm2pgsql/issues