Update README.md for usage and design explaination.

[Update] Establish LKBroadcaster

1. LKNotification protocol for modeling interface for a simple notification
2. LKAnyNotification as concrete type for LKNotification protocol
3. LKObserver for internal usage: Modeling the observer with callback and which operation queue should the callback be executed.
4. LKAnyNotificationCancellable as return type after broadcaster is observed. It can manager the observer should keep receiving notification and not.
5. LKBroadcaster response for post and receive observer for specific notification.