scripts: relnotes: Add reminder to fixup Changes section
scripts: relnotes: Add reminder to fixup Changes section

The Changes section, if left unedited, is pretty much just a copy
of the shortlog. Add a reminder in the auto-generated text that
this section should be edited down to a human readable overview.

Fixes: #517

Signed-off-by: Graham Whaley <95f36e5ac6982f4574a5c449c37cf652e934d2b9@intel.com>

clearcontainers/runtime,chavafg/runtime,devimc/runtime,clearcontainers/runtime,chavafg/runtime,devimc/runtime