Document steps to setup desimodules outside of /global/common/software/desi at NERSC

[dmargala]

After installing desiconda + desi modules in a custom directory, in my $SCRATCH dir at NERSC for example, I would like to use the typical source path/to/desi_environment.sh [release] to load that environment. It may be helpful to add some instructions here or in the desiconda repo how to augment a fresh install of desiconda with this package.

Looking at desi_environment.sh and main files, I see there are some hardcoded paths to /global/common/software/desi that I will probably want to redirect to my custom desiconda + desimodules install.