Adding pm2 example on README.md and local_ip support

[HelmerBarcos]

Hi @bchiang7 I just made following changes. I think they could be very useful.

1. Adding localhost and local_ip support

When trying to run the gatsby server on a VM is mandatory to set the localhost flag. Whit this flag set to 0.0.0.0 we are able to reach the app on http://localhost:PORT and http://LOCAL_IP:PORT.

Taken from https://www.gatsbyjs.com/docs/tutorial/part-1/#run-your-site-locally

Note: If you are using VM setup like vagrant and/or would like to listen on your local IP address, run gatsby develop --host=0.0.0.0. Now, the development server listens on both http://localhost and your local IP.

2. Adding pm2 execution command example for background mode