Add nf-schema comment to the plugin import and sample sheet channel generation

[ramirobarrantes]

Description of feature

I am creating a new pipeline and just became aware of nf-schema (https://nextflow-io.github.io/nf-schema/latest/). The suggestion has come up to add a code comment above the plugin import and sample sheet channel generation with links to the nf-schema documentation website so that people become aware of it.

[ewels]

Link to Slack discussion here.