Add example for cluster mode in heroku integration

[bhumijgupta]

Currently the heroku integration docs does not show how to setup cluster mode for heroku.

How is cluster mode for heroku different?

If we specify instances: "max" in our ecosystem.config.js file, 8 different instances of process would be deployed. But these 8 CPUs as detected by PM2 are shared and the deployment will fail. The better option is to use instances: process.env.WEB_CONCURRENCY || "max".