feat(mixin): add usage

@xen-orchestra/mixin/.USAGE.md

@@ -0,0 +1,20 @@
+- mixins can depend on each other, they will be instanciated on-demand
+
+```js
+import mixin from '@xen-orchestra/mixin'
+
+class MyMixin {
+  constructor(app, ...mixinParams) {}
+
+  foo() {}
+}
+
+class App {
+  constructor() {
+    mixin(this, { MyMixin }, [...mixinParams])
+  }
+}
+
+app = new App()
+app.myMixin.foo()
+```

@xen-orchestra/mixin/README.md

@@ -12,6 +12,29 @@ Installation of the [npm package](https://npmjs.org/package/@xen-orchestra/mixin
 > npm install --save @xen-orchestra/mixin
 ```
 
+## Usage
+
+- mixins can depend on each other, they will be instanciated on-demand
+
+```js
+import mixin from '@xen-orchestra/mixin'
+
+class MyMixin {
+  constructor(app, ...mixinParams) {}
+
+  foo() {}
+}
+
+class App {
+  constructor() {
+    mixin(this, { MyMixin }, [...mixinParams])
+  }
+}
+
+app = new App()
+app.myMixin.foo()
+```
+
 ## Contributions
 
 Contributions are _very_ welcomed, either on the documentation or on