Update Objective-C.gitignore

Adding a comment in case one wants to ignore .xcworkspace

Objective-C.gitignore

@@ -35,6 +35,9 @@ xcuserdata/
 # https://guides.cocoapods.org/using/using-cocoapods.html#should-i-check-the-pods-directory-into-source-control
 #
 # Pods/
+#
+# Add this line if you want to avoid checking in source code from the Xcode workspace
+# *.xcworkspace
 
 # Carthage
 #