Fill out some more documentation about building RenderBox subclasses.

sky/specs/style-guide.md

 Sky Style Guide
 ===============
 
 In general, follow our [Design Principles](design.md) for all code.
 
 The primary goal of this style guide is to improve code readability so
 that everyone, whether reading the code for the first time or
 maintaining it for years, can quickly determine what the code does. A
 secondary goal is avoiding arguments when there are disagreements.
 
@@ skipping 103 matching lines @@
 > For example,
 > ```dart
 >    new EdgeDims.symmetric(horizontal: 8.0);
 > ```
 > ...rather than:
 > ```dart
 >    new EdgeDims(0.0, 8.0, 0.0, 8.0);
 > ```
 
 
+Use for-in loops rather than forEach() where possible, since that
+saves a stack frame per iteration.
+
+
 C++
 ---
 
+Put spaces around operators in expressions.
+
 
 Java
 ----