doc(help): Add section for help extensions

README.md

@@ -143,7 +143,6 @@ Keybinding           | Description
 <kbd>C--</kbd>       | Decrease font size (`text-scale-decrease`).
 <kbd>M-C-l</kbd>     | Switch back and forth between the 2 top buffers (from XEmacs).
 <kbd>C-c C-SPC</kbd> | Toggle highlight of the symbol under the cursor (by default, up to 8 different symbols using different colors).
-<kbd>C-o</kbd>       | Display a transient with bindings for a `casual` user of `org-agenda`, `bookmark`, `calc`, `calendar`, `dired`, `ibuffer`, `info`, `isearch`, `rebuilder`, `symbol-overlay` or a Casual EditKit menu.
 
 Editing:
 
@@ -209,6 +208,29 @@ Keybinding                       | Description
 
 You can change the modifier key (default: <kbd>M</kbd>) by customizing `exordium-highlight-symbol-map-modifier`.
 
+## Help extensions
+Exordium provides a few help extensions that enhance default Emacs
+configuration presenting user with more information. All of them are enabled by
+setting `exordium-help-extensions` to t. Noteably, the following are turned on:
+
+- [which-key](https://github.com/justbur/emacs-which-key) - display available
+  keybindings after a short while (for example type <kbd>C-x</kbd> and wait a
+  second,
+- [casual](https://github.com/kickingvegas/casual) - a collection of
+  opinionated keyboard-driven user interfaces for various built-in Emacs modes,
+- [helpful](https://github.com/Wilfred/helpful) - an alternative to the
+  built-in Emacs help that provides much more contextual information.
+
+Keybinding          | Description
+------------------- |----------------------------------------------------------
+<kbd>C-o</kbd>      | Display a transient with bindings for a `casual` user of `org-agenda`, `bookmark`, `calc`, `calendar`, `dired`, `ibuffer`, `info`, `isearch`, `rebuilder`, `symbol-overlay` or a Casual EditKit menu.
+<kbd>C-h f</kbd>    | Show `helpful` buffer for function, macro, or a special form (`helpful-callable`).
+<kbd>C-h F</kbd>    | Show `helpful` buffer for function (`helpful-function`).
+<kbd>C-h v</kbd>    | Show `helpful` buffer for variable (`helpful-variable`).
+<kbd>C-h C</kbd>    | Show `helpful` buffer for interactive command (`helpful-command`).
+<kbd>C-c C-d</kbd>  | Show `helpful` buffer for thing at point, when in `emacs-lisp-mode` (`helpful-thing-at-point`).
+<kbd>C-j</kbd>      | Show `helpful` buffer for currently selected candidate, when completing read for `helpful` commands.
+
 ## Projectile
 
 [Projectile](https://github.com/bbatsov/projectile) adds the notion of