Describe modules

doc/modules/index.md

@@ -10,6 +10,45 @@ require extra steps to enable all functionality. These steps and the
 functionality provided by these modules are documented on the
 following links.
 
+## What's a module?
+
+Prelude modules are plain old Elisp libraries - there's absolutely nothing magical about them.
+Most of them simply install a few package and provide some sensible baseline configuration for them.
+Here's a real example.
+
+``` emacs-lisp
+;;; prelude-ruby.el --- Emacs Prelude: A nice setup for Ruby (and Rails) devs.
+;;
+;;; Code:
+
+(require 'prelude-programming)
+
+(prelude-require-packages '(inf-ruby yari))
+
+;; We never want to edit Rubinius bytecode
+(add-to-list 'completion-ignored-extensions ".rbc")
+
+(define-key 'help-command (kbd "R") 'yari)
+
+(with-eval-after-load 'ruby-mode
+  (defun prelude-ruby-mode-defaults ()
+    (inf-ruby-minor-mode +1)
+    ;; CamelCase aware editing operations
+    (subword-mode +1))
+
+  (setq prelude-ruby-mode-hook 'prelude-ruby-mode-defaults)
+
+  (add-hook 'ruby-mode-hook (lambda ()
+                              (run-hooks 'prelude-ruby-mode-hook))))
+
+(provide 'prelude-ruby)
+;;; prelude-ruby.el ends here
+```
+
+To use a module you simple have to require it. No new concepts. No magic.
+
+## List of Modules
+
 - C/C++
 - Clojure
 - CoffeeScript