Update usage in README

[AgainPsychoX]

Currently usage described in README makes the app fail if building for production (no dev deps).

In my opinion, one of following should be applied:

1. Mention the behaviour for clarity (even while it's close to obvious),
2. Change the installation to install as regular dependency (no -D/--save-dev)
3. Introduce dynamic loading example, where the extensions are only installed while in development (maybe using isDev flag from electron-is-dev, or app.isPackaged).

---

For inspiration, in my codebase I use something like:

app.whenReady().then(async () => {
	if (isDev) {
		const { default: installExtension, REACT_DEVELOPER_TOOLS } = await import('electron-devtools-installer');
		await installExtension(REACT_DEVELOPER_TOOLS);
	}

	createWindow();
	//...

[jrohlandt]

@AgainPsychoX thanks. I'm using the package based off your example above.

Just out of curiosity: Why do you recommend in step number two to not install the package as a dev dependency? Shouldn't dynamically loading the package only in dev mode prevent any issues in packaged applications? Or is there something else to take into consideration as well?