README update

[heckstra]

In the README, quickstart step 2 provides this db URL:
export DATABASE_URL=postgres://username:password@localhost:5432/fwapg

I got this error:
sqlalchemy.exc.NoSuchModuleError: Can't load plugin: sqlalchemy.dialects:postgres`

After some googling, it worked when I changed postgres:// to postgresql:// in the DATABASE_URL

[smnorris]

Thank you! Good to have more eyes on this.