ROCker build on non-uniprot sequences

[nick-youngblut]

The README.md states:

Collect a good reference collection of the gene of interest. This is the most important step, but there are some resources to help you. In general, we find the resources at UniProt very useful.

... which makes it seem like UniProt is suggested but optional. However, the documentation for ROCker build states that a list of "UniProtKB ID[s]" must be provided via the -p or -P options. So, can ROCker build be used on non-UniProtKB sequences or is UniProtKB a requirement?