update readme slightly, reduce size of preview gif
update readme slightly, reduce size of preview gif

brysgo/react-native-facebook-login,lwhiteley/react-native-facebook-login,leonacky/react-native-facebook-login,brysgo/react-native-facebook-login,leonacky/react-native-facebook-login,lwhiteley/react-native-facebook-login,leonacky/react-native-facebook-login,lwhiteley/react-native-facebook-login,brysgo/react-native-facebook-login