Add a note about production mode for npm installs.

https://github.com/facebook/react/issues/1772
Since we list `envify` as a dependency and [list the transform in package.json](https://github.com/facebook/react/blob/master/npm-react/package.json#L33-L35) - no additional work is needed by consumers. Nobody has to go learn about browserify transforms or anything about envify. Your wording implies there's an exercise for the reader.