improve landing page text

Author: gordonwoodhull

welcome.md

@@ -2,13 +2,9 @@
 
 Welcome to the dc.js documentation.
 
-The entire library is scoped under {@link dc the dc namespace}.
+The entire library is scoped under {@link dc the dc namespace}. Its child namespaces contain utilities.
 
-The other Namespaces contain utilities.
-
-The charts are listed under Classes.
-
-And shared chart functionality is under Mixins.
+A class diagram is shown below - mixins are blue and chart classes are green. (Relations between mixins are somewhat subjective.)
 
 <img src="class-hierarchy.svg"></img>