Work on docs a little bit.

doc/site/classes.markdown

@@ -1,3 +1,28 @@
 ^title Classes
 
-**TODO**
+Every value in Wren is an object, and every object is an instance of a class.
+Even `true` and `false` are full-featured objects, instances of the `Bool` class.
+
+## Defining a class
+
+Classes are created using the `class` keyword, unsurprisingly:
+
+    :::wren
+    class Unicorn {}
+
+This creates a class named `Unicorn` with no methods or fields.
+
+**TODO: methods**
+
+## Inheritance
+
+A class can inherit from a "parent" or *superclass*. When you invoke a method on an object of some class, if it can't be found, it walks up the chain of superclasses looking for it there.
+
+By default, any new class inherits from `Object`, which is the superclass from which all other classes ultimately descend. You can specify a different parent class using `is` when you declare the class:
+
+    :::class
+    class Pegasus is Unicorn {}
+
+This declares a new class `Pegasus` that inherits from `Unicorn`.
+
+**TODO metaclasses, supercalls, fields, etc.**