Add docs for Scheduler class (#902)

I think it's time we had a go at this.

doc/site/cli/modules/scheduler/scheduler.markdown

@@ -1,7 +1,27 @@
 ^title Scheduler Class
 
-**TODO**
+The Scheduler class maintains a list of fibers, to be started one after the other, when a signal to do so is received. The signal (a private method call) is typically transmitted by _long running_ methods in the File or Timer classes which suspend the current fiber so that Wren can carry out other tasks in the meantime.
 
-## Methods
+## Static Method
 
-**TODO**
+### Scheduler.**add**(callable)
+
+Adds a new fiber to the scheduler's fibers list. This fiber calls `callable` and then transfers to the next fiber in the list, if there is one.
+
+`callable` is a function or other object which has a call() method.
+
+<pre class="snippet">
+var a = 3
+
+Scheduler.add {
+  a = a * a
+}
+
+Scheduler.add {
+  a = a + 1
+}
+
+System.print(a)        // still 3
+Timer.sleep(3000)      // wait 3 seconds
+System.print(a)        // now 3 * 3 + 1 = 10
+</pre>