Merge pull request #1540 from visualize-admin/docs/vercel-dev-db

docs: Add bit on migrations & dev database

Author: ptbrowne

README.md

@@ -90,7 +90,7 @@ yarn dev:rollup
 
 #### Database migrations
 
-Database migrations are run automatically when the _production_ app starts. In _development_, you'll have to run them manually:
+Database migrations are run automatically when a production build of the app starts. In _development_, you'll have to run them manually:
 
 ```sh
 yarn db:migrate:dev
@@ -100,6 +100,18 @@ Migrations are located in `db-migrations/`. Write SQL or JS migrations and follo
 
 For detailed instructions, please refer to the [postgres-migrations](https://github.com/thomwright/postgres-migrations) documentation.
 
+⚠️ On Vercel environments like "preview" and "production", "production" build are started which means that database migrations are executed.
+Since all environments are sharing the same database, it means that a database migration executing on 1 database could be disruptive to
+other preview deployments. For example adding a column to the schema would be disruptive, since other preview deployments would try to
+remove it (since the column is not yet in the schema). To prevent any problems on preview deployments, we have a second database that
+is special for development and that must be used if you are working on a branch that brings in database changes. You can configure
+this in the Vercel environment variables by copy-pasting the environment variables found in the [visualization-tool-postgres-dev][]
+storage (see `.env.local` tab), and copy paste them as [environment variables](https://vercel.com/ixt/visualization-tool/settings/environment-variables)
+in the visualisation-project. Take care of scoping the new environment variables to the preview branch you are working on.
+After merging the branch, you can delete the environment variables scoped to the branch.
+
+[visualization-tool-postgres-dev](https://vercel.com/ixt/visualization-tool/stores/postgres/store_dV3rog1asOXO3BfC/data)
+
 ## Versioning
 
 New versions of `package.json` are built on GitLab CI into a separate image that will be deployed to the integration env.