Document module system slightly.
Document module system slightly.

diku-dk/futhark,diku-dk/futhark,HIPERFIT/futhark,diku-dk/futhark,ihc/futhark,ihc/futhark,HIPERFIT/futhark,diku-dk/futhark,diku-dk/futhark,ihc/futhark,HIPERFIT/futhark