Update usage docs for #889

Author: drpatelh

docs/usage.md

@@ -86,7 +86,9 @@ The `--umitools_grouping_method` parameter affects [how similar, but non-identic
 
 ## Reference genome files
 
-The minimum reference genome requirements are a FASTA and GTF file, all other files required to run the pipeline can be generated from these files. However, it is more storage and compute friendly if you are able to re-use reference genome files as efficiently as possible. It is recommended to use the `--save_reference` parameter if you are using the pipeline to build new indices (e.g. those unavailable on [AWS iGenomes](https://nf-co.re/usage/reference_genomes)) so that you can save them somewhere locally. The index building step can be quite a time-consuming process and it permits their reuse for future runs of the pipeline to save disk space. You can then either provide the appropriate reference genome files on the command-line via the appropriate parameters (e.g. `--star_index '/path/to/STAR/index/'`) or via a custom config file.
+Please refer to the [nf-core website](https://nf-co.re/usage/reference_genomes) for general usage docs and guidelines regarding reference genomes.
+
+The minimum reference genome requirements for this pipeline are a FASTA and GTF file, all other files required to run the pipeline can be generated from these files. However, it is more storage and compute friendly if you are able to re-use reference genome files as efficiently as possible. It is recommended to use the `--save_reference` parameter if you are using the pipeline to build new indices (e.g. custom genomes that are unavailable on [AWS iGenomes](https://nf-co.re/usage/reference_genomes#custom-genomes)) so that you can save them somewhere locally. The index building step can be quite a time-consuming process and it permits their reuse for future runs of the pipeline to save disk space. You can then either provide the appropriate reference genome files on the command-line via the appropriate parameters (e.g. `--star_index '/path/to/STAR/index/'`) or via a custom config file.
 
 - If `--genome` is provided then the FASTA and GTF files (and existing indices) will be automatically obtained from AWS-iGenomes unless these have already been downloaded locally in the path specified by `--igenomes_base`.
 - If `--gff` is provided as input then this will be converted to a GTF file, or the latter will be used if both are provided.