Update serve port flag and add example

[lyzhovnik]

Hi everyone! Thank for this awesome project! I enjoy this a lot and use it in my react projects!

So, I needed to run front-end with serve library with my costume port, I made some reserch about all flags and docs about serve and noticed that frags which are on the CRA website are still working, but they are deprecated and serve lib recommends to use flag -l instead of -p, this discussion is in vercel/serve#382 issue, and also you can see this in serve code:
https://github.com/zeit/serve/blob/85d121e5008783395cc607089f23a5039547f952/bin/serve.js#L258

This pull request updates serve ports and adds example of using costume ports with serve lib

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[iansu]

Thanks!