Update peak caller documentation

[vecerkovakaterina]

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 - add to the software_versions process and a regex to scrape_software_versions.py
  • [ ] 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/clipseq branch on the nf-core/test-datasets repository.
• [ ] Make sure your code lints (nf-core lint .).
• [ ] Ensure the test suite passes (nextflow run . -profile test,docker).
• [ ] 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).

Added supported protocols to the different peak callers in usage.md.