clarifying when ignoring is not required

Global/Xcode.gitignore

@@ -2,11 +2,17 @@
 #
 # gitignore contributors: remember to update Global/Xcode.gitignore, Objective-C.gitignore & Swift.gitignore
 
-## Build generated
+## User settings
+xcuserdata/
+
+## compatibility with Xcode 8 and earlier (ignoring not required starting Xcode 9)
+*.xcscmblueprint
+*.xccheckout
+
+## compatibility with Xcode 3 and earlier (ignoring not required starting Xcode 4)
 build/
 DerivedData/
-
-## Various settings
+*.moved-aside
 *.pbxuser
 !default.pbxuser
 *.mode1v3
@@ -15,9 +21,3 @@ DerivedData/
 !default.mode2v3
 *.perspectivev3
 !default.perspectivev3
-xcuserdata/
-
-## Other
-*.moved-aside
-*.xccheckout
-*.xcscmblueprint