added documentation

it's nothing too novel, okay?

backend/README.md

@@ -8,3 +8,5 @@ This is just an orginizational way of keeping the backend services together (so
 
 - [`public`](./public/README.md) - a RESTful API service
 - [`task`](./task/README.md) - a task scheduler service
+- [`storage`](./storage/README.md) - an internal storage library
+- [`cache`](./cache/README.md) - an internal caching library