Add documentation describing why to use hANCESTRO vs NCIT

[cmungall]

It seems hANCESTRO overlaps a lot with the branch of another ontology in OBO, NCIT:

https://www.ebi.ac.uk/ols/ontologies/ncit/terms?iri=http%3A%2F%2Fpurl.obolibrary.org%2Fobo%2FNCIT_C17049

[aside: I think the name of the root class in NCIT is problematic, see https://github.com/NCI-Thesaurus/thesaurus-obo-edition/issues/60]

I think we should at least have mappings between the two (I can help with this).

But I think it would be useful to have more background documentation on the hancestro page: http://obofoundry.org/ontology/hancestro - what are the relevant advantages of one over the other? Is hANCESTOR only intended for GWAS? Why?

This might help other communities trying to find a vocabulary to use for concepts such as ethnicity: GenomicsStandardsConsortium/mixs#59

It looks like the 2018 paper does not mention
https://genomebiology.biomedcentral.com/articles/10.1186/s13059-018-1396-2

[cmungall]

There is also ethnic group in NCIT: https://www.ebi.ac.uk/ols/ontologies/ncit/terms?iri=http%3A%2F%2Fpurl.obolibrary.org%2Fobo%2FNCIT_C16564

[daniwelter]

Documentation now available in new FAQ section