Complete `usage.md`

[Aratz]

seqinspector/docs/usage.md

Line 9 in 487f081

<!-- TODO nf-core: Add documentation about anything specific to running your pipeline. For general topics, please point to (and add to) the main nf-core website. -->

[FranBonath]

started in PR #124

[FranBonath]

The PR above was only about removing the --genome parameter, which we re-introduced. So PR was closed without merge.