doc: Add guile-commonmark as optional dependency.

* doc/haunt.text ("Requirements"): Add guile-commonmark.
* README ("Requirements"): Add guile-commonmark.

README

@@ -48,6 +48,7 @@ To view your creation, run =haunt serve= and browse to
   Optional:
 
   - guile-reader (for Skribe support)
+  - guile-commonmark (for CommonMark support)
 
 * Building from Git
 

doc/haunt.texi

@@ -165,6 +165,9 @@ The following dependencies are optional:
 @item
 @url{http://www.nongnu.org/guile-reader/, Guile-Reader} version 0.6 or
 later (for Skribe support)
+@item
+@url{https://github.com/OrangeShark/guile-commonmark, guile-commonmark}
+version 0.1 or later (for CommonMark support)
 @end itemize
 
 @node Building