Add migration instructions

migrations/how_to_do_migration.md

@@ -0,0 +1,5 @@
+## How to do a migration
+ - Create `.env` to the root folder
+ - Add the database url to the `.env` file: 
+ `DATABASE_URL=postgres://db_user:password@server_url:PORT/db_name`
+ - Run `npm run migrate up`