isoseg: add comments to manifest generation script
isoseg: add comments to manifest generation script

now that we have isolation segments in play, this script is a bit more
complex, and some comments will be useful for future readers

Signed-off-by: Toby Lorne <527052641d65eef236eeef9545b2acac74c35d57@digital.cabinet-office.gov.uk>

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf