Updated env instructions based on PostgreSQL issues

@mathemagica Can you please review these env instructions changes?
That's a good idea. If the error is coming from seeds.rb and the error message is related to image uploading you can ignore the error, and it likely means you dont have imagemagick + X11 running on your local system. You can fix this error locally by installing these packages, but long term we'll want to move this production image upload step out of db/seeds and into its own rake task we only run on production- no need to have everyone with a dev env reuploading all the profile images every time they set up a local dev env.

I would also include steps on how to check if your local postgres db is running. From the terminal you can check with ps like this:

annlewis@ann-lewis-computer-3:~/pedago/back_royal (player_ui_second_pass)$ ps -efwww | grep -i Postgres93

  501  1343     1   0 Tue05PM ??         0:02.93 /Applications/Postgres93.app/Contents/MacOS/bin/postgres -D /Users/annlewis/Library/Application Support/Postgres93/var -p5432

If you see no processes like this running on your local system, you need to start postgres. I use Postgres.App on my mac and start it explicitly as an application, but if you've installed it with brew you can start it with pg_ctl