Added clarification for nested requests

Added a sub-section in the Requests portion to clarify the usage of nested calls a bit. You did list `posts/1/comments` in the list of routes, but it's kinda left up to the reader to figure out what that means. Hopefully this makes it a bit easier to pick up quickly. References [typicode#72](typicode#72)

Author: therebelrobot

README.md

@@ -83,6 +83,10 @@ Returns default index file or serves `./public` directory.
 GET /
 ```
 
+### Nested calls
+
+Any nested objects need to be available in the root of the object, and reference its parent using `<parent-key>Id: <parent-id-value>`. In the above example, to return any `comments` objects for `post`s that have and `id` of `1` would be `/posts/1/comments`, so long as each `comments` object has a `postId` value.
+
 ## Install
 
 ```bash