Update the README

Update the readme to refer to 9.1+2.0 by default.
Information for older versions remains as an alternative.
Add example osm2pgsql usage

The example osm2pgsql usage is for a full planet and based on reasonable defaults for rendering osm-carto.

The example will periodically require updating to increase the RAM cache and quoted flat nodes size.

Also, be clearer about supporting PBF.
Merge pull request #40 from pnorman/example_docs

Update the README
Merge pull request #42 from pnorman/example_help

Add example osm2pgsql usage