update component and qweb doc

doc/component.md

@@ -1,3 +1,30 @@
 # Component
 
-hey
+Components are the reusable, composable widgets. They are designed to be low
+level, to be declarative, and with asynchronous rendering.
+
+For example:
+
+```javascript
+export class Counter extends Component {
+  inlineTemplate = `
+    <div t-name="counter">
+      <button t-on-click="increment(-1)">-</button>
+      <span style="font-weight:bold">Value: <t t-esc="state.counter"/></span>
+      <button t-on-click="increment(1)">+</button>
+    </div>`;
+
+  state = {
+    counter: 0
+  };
+
+  constructor(parent, props) {
+    super(parent, props);
+    this.state.counter = props.initialState || 0;
+  }
+
+  increment(delta) {
+    this.updateState({ counter: this.state.counter + delta });
+  }
+}
+```