Adding page on modules.

site/src/layouts/application.haml

@@ -24,6 +24,8 @@
             =link '<tt>include</tt> and <tt>extend</tt>', '/includeextend.html'
           %li
             =link 'Callbacks and hooks', '/hooks.html'
+          %li
+            =link 'Modules', '/modules.html'
           %li
             =link 'Type checking and the class tree', '/types.html'
       #footer

site/src/pages/modules.haml

@@ -0,0 +1,41 @@
+:textile
+  h3. Modules
+
+  Ruby modules are collections of methods that can be mixed into classes. The instance methods
+  in a module cannot be accessed from outside it, they can only be used in classes that
+  @include@ the module.
+
+  @JS.Class@ has some functionality to mimick modules. @JS.Module@ creates objects that have
+  one method - an @included@ hook - that can be mixed into classes but cannot have their
+  instance methods modified. This allows you to create secure mixins.
+
+  <pre>
+  var Comparable = new JS.Module({
+    extend: {
+      compare: function(a,b) {
+        return a.compareWith(b);
+      }
+    },
+
+    lt: function(object) {
+      return this.klass.compare(this, object) == -1;
+    },
+    lte: function(object) {
+      return this.klass.compare(this, object) < 1;
+    },
+    gt: function(object) {
+      return this.klass.compare(this, object) == 1;
+    },
+    gte: function(object) {
+      return this.klass.compare(this, object) > -1;
+    },
+    eq: function(object) {
+      return this.klass.compare(this, object) == 0;
+    }
+  });</pre>
+
+  @Comparable@'s instance methods are now not accessible unless the module is mixed into a class:
+
+  <pre>
+  TodoItem.include(Comparable);
+  // Now TodoItem has all the methods defined above</pre>