Document how to re-init DB

doc/db.md

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+# Database
+
+
+It's expected that you'll set up a DB separately from the app. The app uses a proxy
+container to enable it to talk to an external DB through the hostname `db`.
+
+
+## Data model
+
+The data model is managed by SQLAlchemy. The model is at
+`usaon-vta-survey/models/tables.py`
+
+
+### Changes to the data model
+
+We haven't started working on data migrations yet.
+
+For now, it's expected that the database will be re-initialized completely to apply
+changes.
+
+To do that:
+
+```
+./scripts/invoke_in_container.sh db.init
+```
+
+> :warning: Don't forget to set USAON_VTA_SURVEY_VERSION envvar to match the running
+> container (`source VERSION.env`), or a new container will be pulled.