docs: link to CCBR/nf-modules & include example

[kelly-sovacool]

Changes

• link to CCBR/nf-modules
• include CCBR/bwa/mem from that repo as an example

Issues

resolves #25

PR Checklist

(Strikethrough any points that are not applicable.)

• This comment contains a description of changes with justifications, with any relevant issues linked.
• [ ] Write unit tests for any new features, bug fixes, or other code changes.
• [ ] Update docs if there are any API changes.
• [ ] If a new nextflow process is implemented, define the process container and stub.
• [ ] Update CHANGELOG.md with a short description of any user-facing changes and reference the PR number. Guidelines: https://keepachangelog.com/en/1.1.0/