I need to `vim` the file found at `/System/Library/LaunchDaemons/com.apple.pfctl.plist` but the permissions will not allow it. Even when I run `sudo vim /System/Library/LaunchDaemons/com.apple.pfctl.plist` and enter my password, VIM throws me a warning and says that I'm editing a readonly file. Any ideas how to do this?

I'm running OS X 10.11 El Capitain.