typography: add “Features:” section

Add a “Features:” section by taking relevant pieces from the
“Description” supersection.

See: syl20bnr#9476

layers/+emacs/typography/README.org

@@ -2,19 +2,21 @@
 
 * Table of Contents                                         :TOC_4_gh:noexport:
 - [[#description][Description]]
+  - [[#features][Features:]]
 - [[#install][Install]]
 - [[#key-bindings][Key bindings]]
   - [[#typo-mode][Typo Mode]]
   - [[#tildify-mode][Tildify Mode]]
 
 * Description
-This layer provides support for typographic text editing in Spacemacs.  It
-provides modes to automatically insert and cycle among typographic characters:
-
-- [[https://github.com/jorgenschaefer/typoel][Typo Mode]] automatically inserts and cycles among typographic Unicode
-  characters on some keys.
-- Tildify Mode automatically inserts non-breaking spaces where required. (Only
-  available on Emacs 25).
+This layer provides support for typographic text editing in Spacemacs.
+
+** Features:
+  - Modes to automatically insert and cycle among typographic characters
+  - [[https://github.com/jorgenschaefer/typoel][Typo Mode]] automatically inserts and cycles among typographic Unicode
+    characters on some keys.
+  - Tildify Mode automatically inserts non-breaking spaces where
+    required (Only available on Emacs 25).
 
 * Install
 To use this configuration layer, add it to your =~/.spacemacs=. You will need to