docs: Adds note to keep Registry in sync

Author: NeedleInAJayStack

Sources/Units/Unit/DefaultUnits.swift

@@ -1,6 +1,6 @@
 /// Static type containing this package's pre-defined units
 enum DefaultUnits {
-    // MARK: If adding additional units to this list, add corresponding entries in the `Unit+DefaultUnits.swift` file
+    // MARK: If adding units to this list, add corresponding entries in the `Unit+DefaultUnits.swift` & `Registry.swift` files
 
     // MARK: Acceleration