Add instructions for deploying on Digital Ocean VPS

Would like some instructions on using this on Ubuntu 20.04 Droplet with NodeJs. Struggling to get this live. After running `yarn build` (which seems to be killed midway through) and then `yarn start` in production environment, I keep getting this error:

`Could not find a production build in the '/var/www/example.com/.next' directory. Try building your app with 'next build' before starting the production server.`

Would appreciate a tutorial on showing how this can be done.