Add changelog.d folder in Xcode project

[Johennes]

Is your feature request related to a problem? Please describe.

Element iOS includes the changelog.d folder in the Xcode project which is very handy because this way changelog files – which are required for every pull request – can be edited directly from Xcode.

Describe the solution you'd like

Include the folder in project.yml.

Describe alternatives you've considered

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Additional context

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