Add documentation describing workflow design choices

[scwatts]

Overview

• in order to assist accessibility to developers documentation on design choices should be made available
• this should also include other relevant information such as less used Groovy syntax
  • https://github.com/nf-core/oncoanalyser/issues/82
• it has been recommended to include this documentation in an existing file rather that a new file e.g.
  • README.md
  • docs/usage.md
  • under relevant workflow/subworkflow NF files

Initial discussion

1. https://github.com/nf-core/oncoanalyser/pull/29#discussion_r1699758567