Forked off from discussion about package CHANGELOGs in #14.

We need to decide how best to capture CHANGELOGs for packages. Some ideas that have been proposed so far:

• Writing a file at the root of the package's folder,
• Generating them from commit messages,
• Generating them from PR descriptions

[UPDATE] After discussion below and consensus on where and how package CHANGELOGs should be stored, I'm adding a few checklist items below for next steps to implement the agreed-upon changes:

• Create PR to package spec to make CHANGELOG files required: Introducing package CHANGELOGs #131
• Once package spec PR is approved but not yet merged:
  • Update integrations repo with skeleton CHANGELOG files for each integration package: Adding package changelogs integrations#675
  • Inform non-integrations repo package owners about adding CHANGELOG files: email sent
• Once existing packages have been updated with skeleton CHANGELOG files:
  • Merge package spec PR: Introducing package CHANGELOGs #131
  • Create/review/merge PR to update elastic-package with package spec changes: Updating package spec dependency elastic-package#252
  • Create/review/merge PR to update integrations repo with elastic-package changes: Adding package changelogs integrations#675