[icu] Document the upstream merging process Bug: 56358 Change-Id: Idccf21d2e08cccedb25bcdc86916a7e5d5cd5007 Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/icu/+/409094 Reviewed-by: Filip Filmar <fmil@google.com> Reviewed-by: Konstantin Pozin <kpozin@google.com>