[Tile-serving] [osm2pgsql] Add Multi Backend Docs and Sample Configuration (#226)

[Kevin Kreiser]

`building.lua` is in the top level directory. it's referenced in the `sample.multi.json` config file. just an example of using lua (WITH COMMENTS) for the multi backend.

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Reply to this email directly or view it on GitHub:
https://github.com/openstreetmap/osm2pgsql/pull/226#issuecomment-66113261
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