Doc Suggestion: Add example of Handlers for serving individual files#63

First of all, thanks for creating this! It works well and the API is clean and simple. I used it to bundle wasm and js into an example app so that a server and all content is bundled into the app. Some of my content is generated dynamically, so delegating everything to ``` http.FileServer(FS(false)) ``` didn't seem workable. I ended up writing handler functions like the following to allow setting proper content headers. Seems to work very well, though I don't know if that's the cleanest possible way to code it. ``` // fsSendStatic serves the requested file from the ESC FS. func fsSendStatic(w http.ResponseWriter, r *http.Request) { _, err := w.Write(FSMustByte(false, r.URL.Path)) if err != nil { log.Fatalf("Writing file %s returned error: %v", r.URL.Path, err) } } // fsSendStaticJS serves the requested javascript file from the ESC FS. func fsSendStaticJS(w http.ResponseWriter, r *http.Request) { w.Header().Set("Content-Type", "application/javascript") fsSendStatic(w, r) } // fsSendStaticWasm serves the requested WebAssembly file from the ESC FS. func fsSendStaticWasm(w http.ResponseWriter, r *http.Request) { w.Header().Set("Content-Type", "application/wasm") fsSendStatic(w, r) } ``` It took a bit of trial and error to get it working. Just wondering if it might save others some time if the README included an example showing how to serve a mixture of static and dynamic content.