Adding more documentation

[mnarayan]

References to

• literature on confounding in general and types of bias that are not due to confounding
• Recent methods in the functional genomics literature such as RUV-4 and SCmerge
• Papers evaluating harmonization methods and whether or not they succeed at de-confounding.
  https://pubmed.ncbi.nlm.nih.gov/22101192/
  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4679071/