Add Nodemon usage instructions for live server restarting

[Melkzedk]

Added instructions for using Nodemon to restart the Node.js server automatically during development.

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[efekrskl]

Thanks for the PR!

Unfortunately, I see several issues with the PR and I don't think it'd be a good contribution. Just to name a few:

1. I don't understand how is this relevant to express-generator docs (at best, it'd be a different section in docs)
2. Now that node --watch is stable, this might be favored (although, I might be wrong, not sure if they map 1:1)
3. Your example doesn't even work, as the main entry file is not server.js with express-generator (as shown in the docs)