Added more package-level comments

data/data.go

@@ -1,3 +1,4 @@
+// Package data provides operations to deal with arbitrary data and MIME types.
 package data
 
 import "io"

default/font/defaultfont.go

@@ -1,3 +1,4 @@
+// Package font provides a default font. Its data is entirely self contained.
 package font
 
 import "golang.org/x/image/font/basicfont"

layouts/doc.go

@@ -0,0 +1,2 @@
+// Package layouts provides a set of pre-made layouts.
+package layouts