Start fleshing out the rendering/README.md

sky/sdk/lib/rendering/README.md

-This directory introduces a layout and painting system built on the
-Sky APIs. It depends on Dart primitives, ../base/*, ../mojo/*, and
-../animation/*.
+Sky Rendering
+=============
+
+The Sky render tree is a low-level layout and painting system based on a
+retained tree of objects that inherit from [`RenderObject`](object.dart). Most
+developers using Sky will not need to interact directly with the rendering tree.
+Instead, most developers should use [Sky widgets](../widgets/README.md), which
+are built using the render tree.
+
+Base Model
+----------
+
+The base class for every node in the render tree is
+[`RenderObject`](object.dart), which defines the base layout model. The base
+layout mode is extremely general and can accomodate a large number of more
+concrete layout models that can co-exist in the same tree. For example, the base
+model does not commit to a fixed number of dimensions or even a cartesian
+coordinate system. In this way, a single render tree can contain render objects
+operating in three-dimensional space together with other render objects
+operating in two-dimensional space, e.g., on the face of a cube in the three-
+dimensional space. Moreover, the two-dimensional layout might be partially
+computed in cartesian coordinates and partially computed in polar coordinates.
+These distinct models can interact during layout, for example determining the
+size of the cube by the height of a block of text on the cube's face.
+
+Not entirely free-wheeling, the base model does impose some structure on the
+render tree:
+
+ * Subclasses of `RenderObject` must implement a `performLayout` function that
+   takes as input a `constraints` object provided by its parent. `RenderObject`
+   has no opinion about the structure of this object and different layout models
+   use different types of constraints. However, whatever type they choose must
+   implement `operator==` in such a way that `performLayout` produces the same
+   output for two `constraints` objects that are `operator==`.
+
+ * Implementations of `performLayout` are expected to call `layout` on their
+   children. When calling `layout`, a `RenderObject` must use the
+   `parentUsesSize` parameter to declare whether its `performLayout` function
+   depends on information read from the child. If the parent doesn't declare
+   that it uses the child's size, the edge from the parent to the child becomes
+   a _relayout boundary_, which means the child (and its subtree) might undergo
+   layout without the parent undergoing layout.
+
+ * Subclasses of `RenderObject` must implement a `paint` function that draws a
+   visual representation of the object onto an `RenderObjectDisplayList`. If
+   the `RenderObject` has children, the `RenderObject` is responsible for
+   painting its children using the `paintChild` function.
+
+ * Subclasses of `RenderObject` must call `adoptChild` whenever they add a
+   child. Similarly, they must call `dropChild` whenever they remove a child.
+
+ * Most subclasses of `RenderObject` will implement a `hitTest` function that
+   lets clients query the render tree for objects that intersect with a given
+   user input location. `RenderObject` itself does not impose a particular
+   type signature on `hitTest`, but most implementations will take an argument
+   of type `HitTestResult` (or, more likely, a model-specific subclass of
+   `HitTestResult`) as well as an object that describes the location at which
+   the user provided input (e.g., a `Point` for a two-dimensional cartesian
+   model).
+
+ * Finally, subclasses of `RenderObject` can override the default, do-nothing
+   implemenations of `handleEvent` and `rotate` to respond to user input and
+   screen rotation, respectively.
+
+The base model also provides two mixins for common child models:
+
+ * `RenderObjectWithChildMixin` is useful for subclasses of `RenderObject` that
+   have a unique child.
+
+ * `ContainerRenderObjectMixin` is useful for subclasses of `RenderObject` that
+   have a child list.
+
+Subclasses of `RenderObject` are not required to use either of these child
+models and are free to invent novel child models for their specific use cases.
+
+Parent Data
+-----------
+
+
+Box Model
+---------
+
+
+Bespoke Models
+--------------
+
+
+Dependencies
+------------
+
+ * [`package:sky/base`](../base)
+ * [`package:sky/mojo`](../mojo)
+ * [`package:sky/animation`](../mojo)