OSM Bright documentation questions

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@ajashton In setting up OSM Bright today I ran into a few questions with the documentation in the [OSM Bright Ubuntu quickstart guide](http://www.mapbox.com/tilemill/docs/guides/osm-bright-ubuntu-quickstart/). As promised, these are my notes. (Let me know if this isn't the right place to share them)
### PostgresSQL and PostGIS

Ubuntu versions are out of date
### Imposm

`aptitude` was a confusing command.
### Download & Import OSM data

"Assuming you downloaded the PBF to your Downloads folder,"
What if I didn't? It was somewhat unclear when that would become important.

```
imposm -U postgres -d osm -m /path/to/osm-bright/imposm-mapping.py \
--read --write --optimize --deploy-production-tables
```

it was unclear that `/path/to/osm-bright/` needed to be customized. It was also unclear that I needed to change ``. In my mind, that could have conceivably been another file somewhere.
### Other

It's a little unclear where I'll start next time I want to use OSM Bright with a project, or what exactly I did with imposm. Obviously I don't need to reinstall postgres, but what steps do I get to skip?

Start with the OSM Bright Ubuntu quickstart guide linked in the issue and review its PostgreSQL/PostGIS, Imposm, and Download & Import OSM data sections. Clarify the Ubuntu versions, the aptitude command, which paths and PBF filename users must replace, and which setup steps can be skipped on a later run. Done means a newcomer can follow the guide without those ambiguities.

Written by the indexing model from the issue text.