Added an architecture doc.

Hopefully this might help people understand how stuff fits together a
bit...

Docs/architecture.md

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+# Perspex Architecture
+
+At the highest level, perspex is split up into a "core" and two "subsystems.
+
+* The core is a set of Portable Class Libraries that can run anywhere.
+* The Windowing subsystem is responsible for creating windows, handling input and scheduling timers.
+* The Rendering subsystem is responsible for drawing.
+
+There are currently two Windowing and two Rendering subsystems:
+
+## Windowing Subsystems
+
+* Perspex.Win32 uses the Win32 API (this also works on 64-bit windows).
+* Perspex.Gtk uses the GTK2 toolkit and can be run on both Windows and *nix.
+
+## Rendering Subsystems
+
+* Perspex.Direct2D1 uses Microsoft's Direct2D1 API.
+* Perspex.Cairo uses Cairo for rendering and Pango for text layout.
+
+## Core
+
+The Perspex core is split up into several assemblies. These assemblies are not separate because
+you will want to use them separately - they are separate to maintain separation of concerns and a
+layered architecture. It is fully possible that they will be ILMerged into a single assembly for
+distribution.
+
+The assemblies are as follows, from lowest to highest level:
+
+### Perspex.Base
+
+The main classes in this assembly are `PerspexObject` and `PerspexProperty`.
+
+These are Perspex's versions of XAML's `DependencyObject` and `DepenendencyProperty`. It also 
+defines a `PerspexDispatcher` which is - surprise - our version of XAML's `Dispatcher`.
+
+### Perspex.Animation
+
+The main class in the assembly is `Animatable`.
+
+Allows PerspexProperties to be animated and provides various utilities related to animation.
+
+### Perspex.SceneGraph
+
+The main class in this assembly is `Visual` and its interface `IVisual`.
+
+Defines the "Visual" layer which is a 2D scene graph, with each node being a `IVisual`/`Visual`. 
+Also defines primitives such as `Point`/`Rect`/`Matrix`, plus `Geometry`, `Bitmap`, `Brush` and 
+whatever else you might need in order to draw to the screen.
+
+### Perspex.Styling
+
+The main interface in this assembly is `IStyleable`.
+
+Defines a CSS-like system for styling controls.
+
+### Perspex.Layout
+
+The main class in this assembly is `Layoutable`.
+
+Defines a XAML-like layout system using `Measure` and `Arrange`. Also defines `LayoutManager` which 
+carries out the actual layout.
+
+### Perspex.Interactivity
+
+The main class in this assembly is `Interactive`.
+
+Defines a system of routed events similar to those found in XAML.
+
+### Perspex.Input
+
+The main class in this assembly is `InputElement`.
+
+Handles input from various devices such as `MouseDevice` and `KeyboardDevice`, together with
+`FocusManager` for tracking input focus. Input is sent from the windowing subsystem to 
+`InputManager` in the form of "Raw" events which are then translated into routed events for the 
+controls.
+
+### Perspex.Controls
+
+There are many important classes in this assembly, but the root of them is `Control`.
+
+Finally defines the actual controls. The `Control` class is analogous to WPF's `FrameworkElement`
+whereas the `TemplatedControl` class is our version of WPF's `Control`. This is also where you'll 
+find all of the basic controls you'd expect.
+
+### Perspex.Themes.Default
+
+Defines a default theme using a set of styles and control templates.
+
+### Perspex.Application
+
+The main class in this assembly is `Application`.
+
+This ties everything together, setting up the service locator, defining the default theme etc.
+
+### Perspex.Diagnostics
+
+Adds utilities for debugging perspex applications, such as `DevTools` which provides a Snoop/WPF
+Inspector/Browser DevTools window for inspecting the state of the application.
+