Add docs recommending the use of nf-core/download when using Singularity
Add docs recommending the use of nf-core/download when using Singularity

ewels/NGI-RNAseq,ewels/NGI-RNAseq,ewels/NGI-RNAseq,ewels/NGI-RNAseq,ewels/NGI-RNAseq