Add a note about `NODE_ENV=production` and the default build being in development mode.

Hi,

As far as I understood, Kresus Makefile and build scripts build development assets and not production-ready assets. As per comments in the scripts, I think one should prefix make commands by NODE_ENV=production to build minified, production-ready assets.

It seems production mode is enforced in Kresus itself, then NODE_ENV=production is required only during build of assets (basically in scripts/build.sh) if I understood correctly.

Thanks,