Add warning to start.sh that we're switching to sg
Created by: mrnugget
This is the first step to make sg the default tool to run the Sourcegraph environment locally.
It prints a message saying that in 2 weeks we'll deprecate the start.sh script and move to sg by default.
It also sleeps for 6 seconds to give enough time to read this (and add a little incentive to switch).
Looks like this:
