Update usage for Serve 7

[nickserv]

Serve has some breaking changes. Most of the command line options including -s and -p have been removed or replaced with a config file. For the sake of simplicity I decided to remove info about non-defaults, but we could link to the docs on rewrite configs (which replace -s) somewhere: https://github.com/zeit/serve-handler#rewrites-array

[gaearon]

Oh dear. Now the build output instructions hardcoded into every existing CRA app is wrong.

cc @leo Any chance we could heads up next time? If I recall correctly someone from Zeit sent a PR to us so we start recommending serve. This is quite frustrating for us now.

[gaearon]

To be clear, we can take this in theory, but:

1. There are old majors of react-dev-utils that are a pain to update
2. People who ejected (about half of our users?) will get a pinned version of react-dev-utils and won’t guess they need to bump the version

[gaearon]

Also, a config file is pretty much a showstopper for us. We’d have to move back to pushstate-server I guess.

[gaearon]

I filed vercel/serve#382. I don't think this particular solution works for us, but thank you for sending it.

[leo]

This is now solved, as per this. You can use -l <port> for setting the port and --single (just like before) for getting a rewrite-preset for serving an SPA.

Sorry for the inconvenience we've caused!