docs: Explain migration progress

prisma/docs/how-to-update-prisma-schema.md

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+## How to update Fairhold's Prisma schema
+
+This guide explains how we can update our Prisma schema and manage migrations to the production database. This highlights the steps we need to take within this repo, for a wider primer please see the Prisma docs: https://www.prisma.io/docs/orm/prisma-migrate
+
+
+1. Update `schema.prisma`
+Make a change to this file which describes the change you would like to make (e.g. add column, add table).
+
+2. Generate migration
+Run `npm run prisma:migrate` - this will apply to the change to your local dev database, and generate a migration file in `/prisma/migrations`
+
+> [!TIP]
+> The command `npm run prisma:migrate-create-only` will generate a migration file without running it. This is helpful if you want to add any additional changes, and manually configure the migration. For example when renaming a column Prisma may drop and recreate (thus losing data). Manually generating an `ALTER table UPDATE column` SQL command allows us to work around this. You'll then need to run `npm run prisma:migrate` to apply your changes.
+>
+> Docs: https://www.prisma.io/docs/orm/prisma-migrate/workflows/development-and-production#customizing-migrations
+
+3. Update Prisma client
+Run `npm run prisma:generate` to update your local client. You should now be able to both interact with the updated Prisma client as well as view the change in your local dev database.
+
+4. Open PR, merge to `main`
+From here, we can open the changes up to peer-review on GitHub. Once approved and merged, CI will apply our new migration to the production database using the "Deploy migrations" action (dir: `/.github/workflows/deploy-migrations.yaml`).
+