Document FXIOS-16744 [Build Instructions] Note the Xcode Swift macro approval prompt

[BrettThePark]

📜 Tickets

Jira ticket
Github issue

💡 Description

Xcode requires an explicit approval before it will build the ModifiedCopy Swift macro, so the first build after a fresh clone stops on a Trust & Enable dialog. Nothing in the README mentions it, and dismissing it fails the build with a macro validation error that reads like a broken checkout rather than a pending approval.

Adds one line to the build steps so the prompt is expected. It also notes the prompt can return later, since the approval is tied to the resolved package version.

No change to focus-ios/README.md — Focus does not depend on the macro.

Request for someone with wiki access: the same note would help on Automated Project Setup with FXIOS, where the page hands off to opening Xcode. fxios passes -skipMacroValidation, so nothing prompts during automated setup itself — the dialog only appears later on the first Cmd+R. I don't have wiki access.

📝 Checklist

• I filled in the ticket numbers and a description of my work
• I updated the PR name to follow our PR naming guidelines
• If needed, I updated documentation and added comments to complex code