Add documentation for using mux to serve a SPA

[fharding1]

Fixes #464

Summary of Changes

1. Adds documentation to the README.md for using mux to serve API endpoints alongside a SPA.

Testing

I ran create-react-app, made a prod build, copied the build directory, and tested this out. Seems to work great.

[elithrar]

It might be more accessible to make this middleware or more self-contained http.HandlerFunc?

e.g.

func ServeJavaScriptApp(staticDir string, entryPoint string) http.Handler {
    // fn := func(w http.ResponseWriter, r *http.Request) { ... }
    //
    // return http.HandlerFunc(fn)
}

... and then:

serveJSHandler := ServeJavaScriptApp("./dist", "index.html")
r.Use(staticHandler)
r.PathPrefix("/").Handler(serveJSHandler)

Not tied to this - just might be more accessible to a newcomer? Open to feedback on this.

[fharding1]

Hmm. I can see how a http.HandlerFunc might be easier for a beginner than a type implementing http.Handler, however I think the overhead of the closure might outweigh that. Overall I think both are about as easy for a beginner to understand (also open to feedback of course 😄).

[elithrar]

So, thinking on this: should we add a “ServeStatic” or similar handler as a built-in? This has come up a few times.

[elithrar]

Options:

a) leave this as docs
b) add a new built-in handler

[fharding1]

I think implementing a built-in handler in gorilla/mux for the few people trying to use the wrong tool for the job (gorilla/mux to serve an SPA—maybe it fits somewhere else in the gorilla toolkit?) wouldn't be worth adding more stuff to the API. Instead, I think it makes sense to leave this as docs so that people will at least stop opening issues asking how to serve an SPA with mux.

[fharding1]

Let me know if you think it still makes sense to add a built-in handler with the docs improvements and I'll implement that 👍

[elithrar]

Let's leave it as-is for now, @fharding1 - we can revisit later. http.FileServer continues to trip users up, so wary of that.

[elithrar]

(Thanks for working on this, too!)

[fharding1]

Np, thank you!