Mention NODE_ENV being what decided prod/dev build.

[armoredelephant]

In the documentation on modes here, there is no mention of NODE_ENV being what determines a production or development build. It is shown in the screenshot but there's no mention in the documentation. I looked over this several times without realizing the importance of the NODE_ENV line from the screenshot. This was definitely an oversight by me, but I think adding a line about NODE_ENV could potentially save someone else the struggle I went through yesterday when trying to get other [modes] to build a production app.