docs: add explainations to generates migrations with TypeORM (#1177)

CONTRIBUTING.md

@@ -101,6 +101,46 @@ We use [Weblate](https://jellyseerr.borgcube.de/projects/jellyseerr/jellyseerr-f
 
 <a href="https://jellyseerr.borgcube.de/engage/jellysseerr/"><img src="https://jellyseerr.borgcube.de/widget/jellyseerr/multi-auto.svg" alt="Translation status" /></a>
 
+## Migrations
+
+If you are adding a new feature that requires a database migration, you will need to create 2 migrations: one for SQLite and one for PostgreSQL. Here is how you could do it:
+
+1. Create a PostgreSQL database or use an existing one:
+
+```bash
+sudo docker run --name postgres-jellyseerr -e POSTGRES_PASSWORD=postgres -d -p 127.0.0.1:5432:5432/tcp postgres:latest
+```
+
+2. Reset the SQLite database and the PostgreSQL database:
+
+```bash
+rm config/db/db.*
+rm config/settings.*
+PGPASSWORD=postgres sudo docker exec -it postgres-jellyseerr /usr/bin/psql -h 127.0.0.1 -U postgres -c "DROP DATABASE IF EXISTS jellyseerr;"
+PGPASSWORD=postgres sudo docker exec -it postgres-jellyseerr /usr/bin/psql -h 127.0.0.1 -U postgres -c "CREATE DATABASE jellyseerr;"
+```
+
+3. Checkout the `develop` branch and create the original database for SQLite and PostgreSQL so that TypeORM can automatically generate the migrations:
+
+```bash
+git checkout develop
+pnpm i
+rm -r .next dist; pnpm build
+pnpm start
+DB_TYPE="postgres" DB_USER=postgres DB_PASS=postgres pnpm start
+```
+
+(You can shutdown the server once the message "Server ready on 5055" appears)
+
+4. Let TypeORM generate the migrations:
+
+```bash
+git checkout -b your-feature-branch
+pnpm i
+pnpm migration:generate server/migration/sqlite/YourMigrationName
+DB_TYPE="postgres" DB_USER=postgres DB_PASS=postgres pnpm migration:generate server/migration/postgres/YourMigrationName
+```
+
 ## Attribution
 
 This contribution guide was inspired by the [Next.js](https://github.com/vercel/next.js), [Radarr](https://github.com/Radarr/Radarr), and [Overseerr](https://github.com/sct/Overseerr) contribution guides.