Add docs/development section for tips on contributors adding new modes

[keiran-rowell-unsw]

Newer pipeline releases will have further 'value-add' features (accessible metrics, .mmcif, MultiQC) beyond base execution of the container.

The nf-test suites will act as contractual validators that the required info is being provided by any new --mode

For any contributoring developer (range of backgrounds), the 'contributing new modes may need to be expanded to include how to pass non-autodetected info into versions.yml, etc.

If it becomes details I'd advocate separating off docs/usage from docs/development, while keeping the fantastic per-mode usage exposed to the nf-core public page.