Add metrics docs

[egor-dolzhenko]

This pull request will:

• Add description of quality metrics
• Move method overview from README.md into a separate document
• Add a link to Andreas' Review BAMs script
• Add citation to the REViewer preprint

Note: Local haplotype phasing output remains undocumented. I think it would be useful to discuss the output format and a more interpretable scoring function before writing the docs.