Updated README.md
Updated README.md

Added one more example to show the user how to remove the observer. This method is super helpful for apps where the DAKeyboardControl is used on various different screens, including standard VCs, modals, SVCs, etc.
cnbin/DAKeyboardControl,jivesoftware/DAKeyboardControl,AlexanderMazaletskiy/DAKeyboardControl,danielamitay/DAKeyboardControl,gaurav1981/DAKeyboardControl,Rusik/DAKeyboardControl