[CT-2586] [Spike] Automatic message for first time contributors

[dbeatty10]

Big idea: provide a short message to first-time contributors so they know what to expect in the pull request review process.

Context: https://dbt-labs.slack.com/archives/C034EV1RH4Z/p1684335803177419?thread_ts=1684173995.312139&cid=C034EV1RH4Z