README: Update instruction for customization

[Jieran-S]

Update the instruction on how to add a new method/dataset/metrics etc. They should cover:

• File, input and output format
• optargs instruction
• modification on the workflow configs (What to add to path/excute config for it to work)

[heylf]

Done and ready for review.

[Jieran-S]

Thank you Florian! That's so fast! I will keep it open just for now and see if we need to add anything after I merged the consensus part into the workflow