docs(repository): explain usage of RepositoryMixin

packages/repository/README.md

@@ -353,6 +353,24 @@ ctx.bind('dataSources.memory').to(ds);
 ctx.bind('repositories.noteRepo').toClass(MyNoteRepository);
 ```
 
+#### Using the Repository Mixin for Application
+A Repository Mixin is available for Application that provides convenience methods for binding and instantiating a repository class. Bound instances can be used anywhere in your application using Dependency Injection. An array set to the key `repositories` can be passed to the constructor or the `.repository(RepositoryClass)` function can be used. The mixin will also instantiate any repositories declared by a component in its constructor using the `repositories` key. 
+
+Repositories will be bound to the key `repositories.RepositoryClass` where `RepositoryClass` is the name of the Repository class being bound. 
+```ts
+import { Application } from '@loopback/core';
+import { RepositoryMixin } from '@loopback/repository';
+import { ProductRepository, CategoryRepository } from './repository';
+
+// Using the Mixin
+class MyApplication extends RepositoryMixin(Application) {}
+
+// MyRepository will be bound to key `repositories.ProductRepository`
+const app = new MyApplication({repositories: [ProductRepository]});
+// MyRepository2 will be bound to key `repositories.CategoryRepository`
+app.repository(CategoryRepository);
+```
+
 ### Compose repositories and controllers in a context
 
 ```ts