Use commonmark-reader.

README.org

@@ -7,7 +7,7 @@ Here’s how to get Haunt and build the site:
 
 #+BEGIN_SRC sh
 cd /path/to/this/dir
-guix package -i haunt
+guix environment --ad-hoc haunt guile-commonmark
 haunt build
 #+END_SRC
 

haunt.scm

@@ -1,6 +1,6 @@
 ;; -*- geiser-scheme-implementation: guile -*-
 ;;; Bootstrappable.org website
-;;; Copyright © 2016 Ricardo Wurmus <rekado@elephly.net>
+;;; Copyright © 2016, 2018 Ricardo Wurmus <rekado@elephly.net>
 ;;;
 ;;; This file is part of the Bootstrappable.org website.
 ;;;
@@ -28,6 +28,7 @@
              (ice-9 ftw)    ; file system
              (ice-9 match)  ; match-lambda
              (haunt reader)
+             (haunt reader commonmark)
              (haunt reader skribe)
              (haunt site)
              (haunt asset)
@@ -210,7 +211,7 @@ place them in the directory DEST."
       '((author . "Ricardo Wurmus")
         (email  . "rekado@elephly.net"))
       #:make-slug %make-slug
-      #:readers (list skribe-reader html-reader)
+      #:readers (list skribe-reader html-reader commonmark-reader)
       #:builders (list (lambda _
                          (read-page skribe-reader
                                     "index.skr"
@@ -218,7 +219,9 @@ place them in the directory DEST."
                                     "index.html"))
                        (lambda (args . rest)
                          (wrap-pages "pages" "." default-layout
-                                     (list skribe-reader html-reader)))
+                                     (list skribe-reader
+                                           html-reader
+                                           commonmark-reader)))
                        (lambda _
                          (directory-assets "static" (const #t) "."))
                        (blog #:theme bootstrappable-theme