chore: Mention setting function target env variable when starting dev server using ide

[gregorygaines]

When starting the functions framework from the Intellij ide, the application throws 404 on each request:

function.go

package function

import (
	"fmt"
	"net/http"

	"github.com/GoogleCloudPlatform/functions-framework-go/functions"
)

func init() {
	functions.HTTP("HelloWorld", helloWorld)
}

// helloWorld writes "Hello, World!" to the HTTP response.
func helloWorld(w http.ResponseWriter, r *http.Request) {
	fmt.Fprintln(w, "Hello, World!")
}

cmd/main.go

package main

import (
	"log"
	// Blank-import the function package so the init() runs
	"github.com/GoogleCloudPlatform/functions-framework-go/funcframework"
	_ "example.com/hello"
)

func main() {
	if err := funcframework.StartHostPort("127.0.0.1", "8082"); err != nil {
		log.Fatalf("funcframework.StartHostPort: %v\n", err)
	}
}

When starting a go app from an IDE, either set the "FUNCTION_TARGET" env variable or set manually in code:

package main

import (
	"log"
	// Blank-import the function package so the init() runs
	"github.com/GoogleCloudPlatform/functions-framework-go/funcframework"
	_ "example.com/hello"
)

func main() {
        err := os.Setenv("FUNCTION_TARGET", "HelloWorld")
	if err != nil {
		log.Fatalf("failed to set FUNCTION_TARGET: %v", err)
	}

	if err := funcframework.StartHostPort("127.0.0.1", "8082"); err != nil {
		log.Fatalf("funcframework.StartHostPort: %v\n", err)
	}
}

This PR adds commented code to the quickstart example mentioning that local servers started from IDE's require the function target env variable set to the function name.