Update README.md to include cmd descriptions

[zztin]

Some options are not included in the original README.md such as the -g group_file function of somalier relate. Including the complete cmd line help message to enable easier searching.

• Detail changes:
  • Include all help messages from somalier subcommands
  • VCF section was merged into previous section after usage for .bcf
• Other changes that can be done:
  • Currently, the # Usage paragraph appeared twice. Could be merged into one.
  • somalier ancestry' help message remain as somalier pca. Could be changed to somalier ancestry for consistency.

[brentp]

I put an ancestry wiki here: https://github.com/brentp/somalier/wiki/ancestry

[zztin]

The README.md is updated as we discussed. It's ready for review.