Add instructions on how to set up App Store signing

[kitallis]

• Ensure your app builds locally if you're changing the bundle identifier
• Ensure you have access to the keys tab in your new account: https://appstoreconnect.apple.com/access/api and create a new key https://docs.tramline.app/integrations/distribution/app_store#generate-an-api-key
• Ensure you can create certificates from https://developer.apple.com/account/resources/certificates/list
• Setup an iOS Distribution (App Store and Ad Hoc) certificate – Certificates are used for code signing
• Setup an App Store Distribution provisioning profile – Provisioning profiles are complements to certificates that hold device / app identifiers that have access to run the app.
• Assign your devices to this said profile
• Replace your Github Actions variables. Refer to this https://docs.tramline.app/integrations/ci-cd/#supported-integrations-1