Add to docs that you can use SRA ftp links with default input csv

[Vlad-Dembrovskyi]

to usage.md change definition of --reads and --download_from parameters. Reads can also take in s3 and ftp links; sownload_from needs to be false not for local but when direct paths or links are provided, not accession numbers.

[Vlad-Dembrovskyi]

https://github.com/TheJacksonLaboratory/splicing-pipelines-nf/blob/33ba660b95d69614dd3f1585c6cf92037128f69c/main.nf#L29