Add docs mirtop

[atrigila]

Improve docs regarding input files from MirgeneDB

PR checklist

• This comment contains a description of changes (with reason).
• If you've fixed a bug or added code that should be tested, add tests!
• If you've added a new tool - have you followed the pipeline conventions in the contribution docs
• If necessary, also make a PR on the nf-core/smrnaseq branch on the nf-core/test-datasets repository.
• Make sure your code lints (nf-core pipelines lint).
• Ensure the test suite passes (nextflow run . -profile test,docker --outdir <OUTDIR>).
• Check for unexpected warnings in debug mode (nextflow run . -profile debug,test,docker --outdir <OUTDIR>).
• Usage Documentation in docs/usage.md is updated.
• Output Documentation in docs/output.md is updated.
• CHANGELOG.md is updated.
• README.md is updated (including new tool citations and authors/contributors).

[nschcolnicov]

Hi @atrigila, I already updated the mirgenedb hairpin file: https://github.com/nf-core/test-datasets/blob/smrnaseq/MirGeneDB/mirgenedb_hsa_hairpin.fa. This was what was causing the issue #477. Apparently we had the wrong links from the start d5500c3.
Or are you referring to something else?

[nschcolnicov]

A suggestion to make it even more clear, regardless if you add it or not, looks good to me!