Updated README.md with some usage information

README.md

@@ -8,6 +8,27 @@
 A [Ladda](https://github.com/ladda-js/ladda) plugin to allow to create
 observables on all `READ` operation.
 
+## How to use
+The observable plugin should work directly with your existing Ladda configuration. However you need to make sure you have the following things configured:
+- API-functions need the operation set to `READ` to be observable.
+- The other API-functions for the same Entity also need operations set for the observable to be notified of changes.
+
+An observable can then be created by calling `createObservable` on the API-function to make it observable. You can subscribe to an observable using `subscribe()`. It takes a callback which is called whenever the entity updates. (The callback is called immediately after subscribing, performing an initial `READ` operation if needed.)
+```javascript
+observable.subscribe(callback);
+```
+
+`subscribe()` returns a disposable reference that you can use to `unsubscribe()` to the observable.
+```javascript
+const disposable = observable.subscribe(callback);
+...
+disposable.unsubscribe();
+```
+
+The `subscribe()` method also takes a second callback which is called when an error occurs.
+```javascript
+observable.subscribe(changeCallback, errorCallback);
+```
 
 ## Contribute