[v2 docs] document `directives`

[scottgigante-immunai]

Nowhere on the Contribute -> Add a method page does it explain to me how to select directives. If I want to use a GPU for my method, how do I do it?

[rcannood]

Good point. The notation would be:

platforms:
  - type: docker
    ...
  - type: nextflow
    directives: 
      label: [ midcpu, midmem, gpu ]

I'll look into where to store this information.