[DOC] event_bus: add initial doc

closes #157

doc/event_bus.md

@@ -0,0 +1,27 @@
+# 🦉 Event Bus 🦉
+
+It is sometimes useful to use a `Bus` to communicate informations between various
+parts of the code. Owl has a very simple bus class, which manages subscriptions,
+triggering events, and callbacks.
+
+```js
+const bus = new owl.EventBus();
+
+bus.on("some-event", null, function(...args) {
+  console.log(...args);
+});
+
+bus.trigger("some-event", 1, 2, 3);
+// [1,2,3] will be logged to the console
+```
+
+Its API is:
+
+| Method                           | Description                       |
+| -------------------------------- | --------------------------------- |
+| `on(eventType, owner, callback)` | add a listener                    |
+| `off(eventType, owner)`          | remove all listeners for an owner |
+| `trigger(eventType, ...args)`    | trigger an event                  |
+| `clear`                          | remove all subscriptions          |
+
+Note that the [`Store`](store.md) is an example of an `EventBus`.

doc/readme.md

@@ -7,6 +7,7 @@
 - [QWeb](qweb.md)
 - [Store](store.md)
 - [Observer](observer.md)
+- [Event Bus](event_bus.md)
 - [Virtual DOM](vdom.md)
 - [Utils](utils.md)
 

doc/store.md

@@ -63,7 +63,7 @@ store.dispatch('addTodo', 'fix all bugs');
 
 ## Reference
 
-The store is a simple `owl.EventBus` that triggers `update` events whenever its
+The store is a simple [`owl.EventBus`](event_bus.md) that triggers `update` events whenever its
 state is changed.  Note that these events are triggered only after a microtask
 tick, so only one event will be triggered for any number of state changes in a
 call stack.