suggest skip pruning at harmonization

[mikeDTI]

Please make sure that this is a documentation issue.

System information

• GenoML Version: 2
• Doc Link: https://genoml.com/docs/guides/harmonization

Describe the documentation Issue

Remind people that they likely want to "--skip_prine yes" as pruning can make the SNP set even smaller. This way if all variables overlap, you can skip retuning and training also.

We welcome contributions by users. Will you be able to fix the Doc issue?