Mention "-scmProvider system" for XCode builds/Swift Package Manager

Co-authored-by: rr-james-hickman <[email protected]>

README.md

@@ -126,6 +126,10 @@ env:
   CARGO_NET_GIT_FETCH_WITH_CLI: true
 ```
 
+### Using Deploy Keys with Swift Package Manager
+
+`xcodebuild` by default uses Xcode's built-it Git tooling. If you want to use GitHub Deploy Keys as supported by this action, however, that version of Git will lack the necessary URL remapping. In this case, pass `-scmProvider system` to the `xcodebuild` command, as mentioned in [Apple's documentation](https://developer.apple.com/documentation/swift_packages/building_swift_packages_or_apps_that_use_them_in_continuous_integration_workflows#3680255).
+
 ## What this Action *cannot* do for you
 
 The following items are not issues, but beyond what this Action is supposed to do.