README: add gracefully shutdown http server example

[amikai]

run provides a good way to write a gracefully shutting down HTTP server. It also provides a SignalHandler to make it more elegant. So I want to add this example in the Readme.

What's changed

• Delete some trailing whitespace in the Readme
• Add gracefully shutdown http server example

[peterbourgon]

SignalHandler is orthogonal to the example code that's included in the README, and in any case the package docs should (hopefully!) make clear how to use that particular helper.

I guess it could make sense to add an http.Server.Shutdown example to the README, but it would probably need to look something like my suggestion, here.

[peterbourgon]

Suggested change

	### Gracefully shutdown HTTP server
	```go
	g := &run.Group{}
	g.Add(run.SignalHandler(context.Background(), syscall.SIGINT, syscall.SIGTERM))
	srv := &http.Server{}
	g.Add(func() error {
	log.Printf("starting server")
	return srv.ListenAndServe()
	}, func(err error) {
	log.Printf("shutting down server")
	shutdownErr := srv.Shutdown(context.Background())
	log.Printf("shutdown err: %v", shutdownErr)
	})
	err := g.Run()
	log.Printf("run err: %v", err)
	```
	### http.Server.Shutdown
	```go
	srv := &http.Server{
	Addr: "localhost:8080",
	Handler: ...,
	}
	g.Add(func() error {
	return srv.ListenAndServe()
	}, func(error) {
	ctx, cancel := context.WithTimeout(context.TODO(), 3*time.Second)
	defer cancel()
	srv.Shutdown(ctx)
	})

[amikai]

SignalHandler is orthogonal to the example code that's included in the README, and in any case the package docs should (hopefully!) make clear how to use that particular helper.

Yes, you’re right—this SignalHandler is orthogonal to the code in Readme. In my opinion, the examples currently only shows the control flow of run but doesn’t demonstrate the helper (SignalHandler).

I’d like to add it to the README because:

1. Some users only skim the README to see if it has the features they need before diving into the package documentation for details. That’s why most authors include real-world examples in the README. For instance, in this case, we could show users that run provides a SignalHandler. I believe most users aren’t aware of this feature—at least my colleagues aren’t.
2. Gracefully shutting down an HTTP server is a commonly used piece of code, and I’d like users to know that run can accomplish this too.

That said, I’m totally okay with your approach as well, and I think it could work just fine. What do you think about proceeding with this?