Document how to add capability

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Added
 - Added `buildArchitectures` to scheme build options and target schemes, to control Xcode's "Override Architectures" scheme setting #1642 @arhxam
+- Added FAQ documentation on how to add an Xcode capability, such as In-App Purchase #1644 @Hokila
 
 ### Changed
 - Generated schemes now default to `buildArchitectures: matchRunDestination` ("Match Run Destination"), matching Xcode's default for new schemes. Previously schemes used each target's architecture settings. Set `buildArchitectures: useTargetSettings` to keep the previous behaviour #1642 @yonaskolb

Docs/FAQ.md

@@ -3,6 +3,7 @@
 - [Can I use CocoaPods](#can-i-use-cocoapods)
 - [Can I use Crashlytics](#can-i-use-crashlytics)
 - [How do I setup code signing](#how-do-i-setup-code-signing)
+- [How do I add an Xcode capability](#how-do-i-add-an-xcode-capability)
 
 ## Can I still check in my project
 Absolutely. You will get the most out of XcodeGen by adding your project to your `.gitignore`, as this way you avoid merge conflicts. But you can also check it in as a halfway step.
@@ -41,3 +42,18 @@ This script will be added after `[CP] Embed Pods Frameworks.`
 ## How do I setup code signing
 
 At the moment there are no special options for code signing in XcodeGen, and this must be configured via regular build settings. For code signing to work, you need to tell Xcode which development team to use. This requires setting the `DEVELOPMENT_TEAM` and possibly `CODE_SIGN_STYLE` build settings. See [Configuring build settings](Usage.md#configuring-build-settings) for how to do that
+
+## How do I add an Xcode capability
+
+XcodeGen does not provide a general-purpose abstraction for Xcode capabilities. To determine what a capability requires, temporarily remove the generated project from source control, enable the capability in Xcode, and inspect the resulting project and entitlements changes.
+
+For example, enabling In-App Purchase adds the StoreKit framework. You can express that dependency in your spec as follows:
+
+```yaml
+targets:
+  MyApp:
+    dependencies:
+      - framework: StoreKit.framework
+```
+
+See [Dependencies](ProjectSpec.md#dependency) for the other dependency forms supported by XcodeGen. This example is specific to In-App Purchase; other capabilities may require different project settings, entitlements, or Xcode project attributes.