Fix README sample code errors

Closes https://github.com/tus/tusd/pull/87

README.md

@@ -54,14 +54,17 @@ your own Go program:
 package main
 
 import (
+	"fmt"
+	"net/http"
+
 	"github.com/tus/tusd"
 	"github.com/tus/tusd/filestore"
-	"net/http"
 )
 
 func main() {
 	// Create a new FileStore instance which is responsible for
 	// storing the uploaded file on disk in the specified directory.
+	// This path _must_ exist before tusd will store uploads in it.
 	// If you want to save them on a different medium, for example
 	// a remote FTP server, you can implement your own storage backend
 	// by implementing the tusd.DataStore interface.
@@ -79,22 +82,23 @@ func main() {
 	// Create a new HTTP handler for the tusd server by providing a configuration.
 	// The StoreComposer property must be set to allow the handler to function.
 	handler, err := tusd.NewHandler(tusd.Config{
-		BasePath:      "files/",
+		BasePath:      "/files/",
 		StoreComposer: composer,
 	})
 	if err != nil {
-		panic("Unable to create handler: %s", err)
+		panic(fmt.Errorf("Unable to create handler: %s", err))
 	}
 
 	// Right now, nothing has happened since we need to start the HTTP server on
 	// our own. In the end, tusd will start listening on and accept request at
 	// http://localhost:8080/files
-	http.Handle("files/", http.StripPrefix("files/", handler))
+	http.Handle("/files/", http.StripPrefix("/files/", handler))
 	err = http.ListenAndServe(":8080", nil)
 	if err != nil {
-		panic("Unable to listen: %s", err)
+		panic(fmt.Errorf("Unable to listen: %s", err))
 	}
 }
+
 ```
 
 Please consult the [online documentation](https://godoc.org/github.com/tus/tusd)