[Docs] Explain how to enable plugins in Xcode and Xcode Cloud (#653)

### Motivation

By default, adopters hit this error both in Xcode and Xcode Cloud and
might not be familiar with how to resolve it.

### Modifications

Expand the FAQ to cover how to enable plugins in Xcode and Xcode Cloud.

### Result

Better docs, adopters can find a solution easier.

### Test Plan

N/A

Sources/swift-openapi-generator/Documentation.docc/Articles/Frequently-asked-questions.md

@@ -71,3 +71,23 @@ For Xcode projects, make sure the target that uses the Swift OpenAPI Generator b
 Alternatively, change the access modifier of the generated code to either `internal` (if no code outside of that module needs to use it) or `public` (if the generated code is exported to other modules and packages.) You can do so by setting `accessModifier: internal` in the generator configuration file, or by providing `--access-modifier internal` to the `swift-openapi-generator` CLI.
 
 For details, check out <doc:Configuring-the-generator>.
+
+### How do I enable the build plugin in Xcode and Xcode Cloud?
+
+By default, you must explicitly enable build plugins before they are allowed to run.
+
+Before a plugin is enabled, you will encounter a build error with the message `"OpenAPIGenerator" is disabled`.
+
+In Xcode, enable the plugin by clicking the "Enable Plugin" button next to the build error and confirm the dialog by clicking "Trust & Enable".
+
+In Xcode Cloud, add the script `ci_scripts/ci_post_clone.sh` next to your Xcode project or workspace, containing:
+
+```bash
+#!/usr/bin/env bash
+
+set -e
+
+defaults write com.apple.dt.Xcode IDESkipPackagePluginFingerprintValidatation -bool YES
+```
+
+Learn more about Xcode Cloud custom scripts in the [documentation](https://developer.apple.com/documentation/xcode/writing-custom-build-scripts).