Fill out some more documentation about building RenderBox subclasses.

sky/sdk/lib/base/README.md

 Sky Base
 ========
 
+AbstractNode
+------------
+
+The [node.dart](node.dart) file defines a class, `AbstractNode`, which
+can be used to build mutable trees.
+
+* When a subclass is changing the parent of a child, it should
+  call either parent.adoptChild(child) or parent.dropChild(child)
+  as appropriate. Subclasses should expose an API for
+  manipulating the tree if you want to (e.g. a setter for a
+  'child' property, or an 'add()' method to manipulate a list).
+
+* You can see the current parent by querying 'parent'.
+
+* You can see the current attachment state by querying
+  'attached'. The root of any tree that is to be considered
+  attached should be manually attached by calling 'attach()'.
+  Other than that, don't call 'attach()' or 'detach()'. This is
+  all managed automatically assuming you call the 'adoptChild()'
+  and 'dropChild()' methods appropriately.
+
+* Subclasses that have children must override 'attach()' and
+  'detach()' as described below.
+
+* Nodes always have a 'depth' greater than their ancestors'.
+  There's no guarantee regarding depth between siblings. The
+  depth of a node is used to ensure that nodes are processed in
+  depth order. The 'depth' of a child can be more than one
+  greater than the 'depth' of the parent, because the 'depth'
+  values are never decreased: all that matters is that it's
+  greater than the parent. Consider a tree with a root node A, a
+  child B, and a grandchild C. Initially, A will have 'depth' 0,
+  B 'depth' 1, and C 'depth' 2. If C is moved to be a child of A,
+  sibling of B, then the numbers won't change. C's 'depth' will
+  still be 2. This is all managed automatically assuming you call
+  'adoptChild()' and 'dropChild()' appropriately.
+
+
 Dependencies
 ------------
 
 No dependencies except for `dart:sky` and Dart's core libraries.