improve instructions for opening devtools

[philipp-schwemberger]

As I experienced today, it is incredibly helpful to have devtools in the built as well.
Stuff that worked (for some incredible reason) in dev-mode didn't work in the built.

So it would be nice to know in the docs of nd that you can enter devtools simply by:

• navigate to the folder of your app
• and run: DEVTOOLS=true ./my app in the console