[Docs] Add note about needing to build SwiftPM-Package scheme first (#5732)

Author: ncooke3

CONTRIBUTING.md

@@ -46,7 +46,9 @@ SwiftPM is typically built with a pre-existing version of SwiftPM present on the
 2. Verify the expected version of Xcode was installed.
 3. Open SwiftPM's `Package.swift` manifest with Xcode.
 4. Use Xcode to inspect, edit, and build the code.
-5. Select the `SwiftPM-Package` scheme to run the tests from Xcode.
+5. Select the `SwiftPM-Package` scheme to run the tests from Xcode. Note that the `SwiftPM-Package`
+should be built prior to running any other schemes. This is so the `PackageDescription` module can be
+built and cached for use.
 
 ### Using the Command Line