Cow does not include new modules in the changelog for their first release

[GuySartorelli]

If we add a new module (e.g. if we add support for a module we previously didn't support) we currently have to add a one-off hard-coded hack to CreateChangelog.php's getChangelogLibrary() method. Otherwise, cow will skip generating the changelog for any module that wasn't in the recipe's previous release.
This caused silverstripe/developer-docs to not be included initially for the 4.12 release, which was the first time it was being included by cow.

Acceptance criteria

• Cow automatically correctly handles modules the first time they are added to a cow release
  or
• The above workaround is clearly documented in our processes