Updates SPM instructions (*)

* Includes requirement of adding the FLEXLAYOUT_SWIFT_PACKAGE definition

Author: gcox

README.md

@@ -1116,7 +1116,8 @@ To integrate FlexLayout into another Swift Package, add it as a dependency:
 
 #### In an Xcode target
 
-To integrate FlexLayout into an Xcode target, use the `File -> Swift Packages -> Add Package Dependency` menu item.
+1. To integrate FlexLayout into an Xcode target, use the `File -> Swift Packages -> Add Package Dependency` menu item.
+2. Add "FLEXLAYOUT_SWIFT_PACKAGE=1" to the Xcode target's `GCC_PREPROCESSOR_DEFINITIONS` build setting.
 
 
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