Better GRCH38 Documentation for input files

[apeltzer]

PR checklist

• PR is made against dev branch
• This comment contains a description of changes (with reason)
• Documentation in docs is updated

This just updates a bit more details on the GRCH38 genome reference building process. I might work on adding the GRCH38 genome to the buildReferences.nf script if you want that... ?

[maxulysse]

Thanks a lot about that.

I don't think the buildReferences.nf script will be used on other genomes than the testing one, as all is planned to be made available via iGenomes.

[apeltzer]

Ok, then we'll wait for iGenomes being available :-)