Document the "random" module.

doc/site/modules/random/index.markdown

@@ -1,5 +1,10 @@
-^title Random Module
+^title Module "random"
 
-**TODO**
+This module provides a simple, fast pseudo-random number generator.
+
+It is an optional module. You can omit it from your application by setting the
+preprocessor constant `WREN_OPT_RANDOM` to `0`.
+
+It contains a single class:
 
 * [Random](random.html)