docs: added version differences between v1 and v2

Author: leonsteinhaeuser

README.md

@@ -9,9 +9,16 @@
 [![issues closed](https://img.shields.io/github/issues-closed/leonsteinhaeuser/observer)](https://img.shields.io/github/issues-closed/leonsteinhaeuser/observer)
 [![license](https://img.shields.io/github/license/leonsteinhaeuser/observer)](https://img.shields.io/github/license/leonsteinhaeuser/observer)
 
-
 This library implements the pub/sub pattern in a generic way. It uses Go's generic types to declare the type of the event.
 
+## Differences between v1 and v2
+
+The v2 release contains breaking changes. The most important ones are:
+
+- The `Observable` interface does not provide a `Clients() int64` method anymore.
+- The constructor `NewObserver()` has been removed. Instead, use `new(observer.Observer[T])`.
+- The `Observer` has become a `NotifyTimeout` that can be used to set a timeout for the `NotifyAll` method. The default value is `5 * time.Second`.
+
 ## Usage
 
 ```bash