Update the README

[Serneum]

• Add heroku run "mix ecto.migrate" when adding new models
• Add heroku run "POOL_SIZE=2 mix run priv/repo/seeds.exs" when we need to reseed data (Can 2 become any number up to 18?)
• Add information on setting up psql:
  • brew install postgres
  • Add information on adding a new superuser in psql: createuser -s admin
  • Set up autostart for psql: mkdir -p ~/Library/LaunchAgents && ln -sfv /usr/local/opt/postgresql/*.plist ~/Library/LaunchAgents
  • Start psql: launchctl load ~/Library/LaunchAgents/homebrew.mxcl.postgresql.plist
• Add creation/setup of db:
  • mix ecto.create && mix ecto.migrate && mix run priv/repo/seeds.exs