asset: Add brief documentation.

* haunt/asset.scm: Add introductory comment.
* README.md (Example Configuration): Add 'directory-assets' example.

example/haunt.scm

@@ -1,5 +1,6 @@
 (use-modules (haunt site)
              (haunt reader)
+             (haunt asset)
              (haunt builder blog)
              (haunt builder atom)
              (srfi srfi-19))