Update README.md

to mention about build front-end bundle on upgrade guide

Author: jackycute

README.md

@@ -64,10 +64,11 @@ If you are upgrading HackMD from an older version, follow these steps:
 1. Fully stop your old server first (important)
 2. `git pull` or do whatever that updates the files
 3. `npm install` to update dependencies
-4. Modify the file named `.sequelizerc`, change the value of the variable `url` with your db connection string
+4. Build front-end bundle by `npm run build:prod` (use `npm run build:dev` if you are in development)
+5. Modify the file named `.sequelizerc`, change the value of the variable `url` with your db connection string
    For example: `postgres://username:password@localhost:5432/hackmd`
-5. Run `node_modules/.bin/sequelize db:migrate`, this step will migrate your db to the latest schema
-6. Start your whole new server!
+6. Run `node_modules/.bin/sequelize db:migrate`, this step will migrate your db to the latest schema
+7. Start your whole new server!
 
 Structure
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