moved project structure notes in

CONTRIBUTING.md

@@ -44,6 +44,9 @@ to ensure all the tests pass.
 
 The two main projects are the `Octokit` and `Octokit.Reactive` projects.
 
+The `Octokit.Reactive` library is a thin wrapper over the `Octokit` 
+library - for those who want to use Reactive Extensions (Rx) instead of tasks.
+
 The namespaces are organised so that the relevant components are easy to discover:
 
  - **Authentication** - everything related to authenticating requests 
@@ -56,6 +59,20 @@ The namespaces are organised so that the relevant components are easy to discove
 Unless you're modifying some core behaviour, the **Clients** and **Models** namespaces
 are likely to be the most interesting areas.
 
+The clients within a project are organized similarly to the endpoints in the
+[GitHub API documentation](http://developer.github.com/v3/)
+
+Some clients are "sub-clients". For example, when you navigate to the
+[Issues API](http://developer.github.com/v3/issues/) you'll notice there's an
+endpoint for issues. But in the right navbar, there are other APIs such as
+[Assignees](http://developer.github.com/v3/issues/assignees/) and
+[Milestones](http://developer.github.com/v3/issues/milestones/).
+
+We've tried to mirror this structure. So the `IObservableMilestoneClient` isn't
+a direct property of `IObservableGitHubClient`. Instead, it's a property of the
+`IObservableIssuesClient`. And thus you can get to it by going to
+`client.Issues.Milestones`.
+
 ### What needs to be done?
 
 We have a [`easy-fix`](https://github.com/octokit/octokit.net/issues?labels=easy-fix&state=open)