add spack instructions to darshan documentation

For example, the following works on cori:

spack install darshan-runtime@develop+slurm darshan-util@develop
module use `spack location -i darshan-runtime`/share/craype-2.x/modulefiles
# note that user-added module paths appear to take precedent over system ones
module load darshan/3.2.1

# compile and run job as usual
# resulting .darshan file is placed in home directory as specified by spack settings