Document gotchas: Airflow DAGs paused; new dataset in warehouse

[lauriemerrell]

As a new contributor to the Cal-ITP warehouse, I need common "gotchas" to be documented so that I can complete all steps to get my functionality added and running as expected.

Specifically, a few things we ran into on #3129 that should be documented somewhere:

• Airflow DAGs will be paused by default when merged and need to be manually unpaused if we want them running
• If you're creating a new mart folder in the dbt warehouse, it needs to be added to the dbt project YAML file or the tables will be created in the staging dataset