Fill out some more documentation about building RenderBox subclasses.

sky/specs/style-guide.md

Index: sky/specs/style-guide.md
diff --git a/sky/specs/style-guide.md b/sky/specs/style-guide.md
index 3fa309c04daf689e4e0e69169981139e897d45ae..9d9a97bb9a9fbab4a75c8d50eb9ebeb1240cdcb4 100644
--- a/sky/specs/style-guide.md
+++ b/sky/specs/style-guide.md
@@ -97,9 +97,15 @@ options.
 > ```
 
 
+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
 ----