Remove GNUmakefile, document cmake building.

The README now suggests using nice and make -j for multicore machines.
Expand instructions for separate build directory

This, hopefully, makes the steps a bit more clear by introducing an
explicit layout example to which the command sequences can relate.
Merge pull request #2 from bska/remove-gnumakefile

Expand instructions for separate build directory
Merge pull request #164 from atgeirr/remove-gnumakefile

Remove confusing GNUmakefile and document cmake building process.