describe some common gotchas in the README

[grahamgower]

Various problems can cause sex determination to be inconclusive (intermediate read dosage):

• check library-level bams, to exclude bioinfo issues with accidentally merging libraries from distinct individuals
• filter reads with low MAPQ (e.g. < 30)
• ensure mito reads are excluded - some samples can have high MT/nuc ratio and if there is no mito sequence in the reference, the mito reads can map to NUMTs instead