asset: Add brief documentation.

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

README.md

@@ -23,7 +23,8 @@ Example Configuration
 (use-modules (haunt site)
              (haunt reader)
              (haunt builder blog)
-             (haunt builder atom))
+             (haunt builder atom)
+             (haunt builder assets))
 
 (site #:title "Built with Guile"
       #:domain "example.com"
@@ -33,7 +34,8 @@ Example Configuration
       #:readers (list sxml-reader html-reader)
       #:builders (list (blog)
                        (atom-feed)
-                       (atom-feeds-by-tag)))
+                       (atom-feeds-by-tag)
+                       (static-directory "images"))
 ```
 
 Usage

example/haunt.scm

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

haunt/asset.scm

@@ -35,6 +35,11 @@
             install-asset
             directory-assets))
 
+;; Assets are static files that are copied verbatim from a site's
+;; source directory to the target output directory, such as images,
+;; CSS, and JavaScript files.  The 'source' and 'target' fields are
+;; file names that are relative to a source and target directory,
+;; respectively.
 (define-record-type <asset>
   (make-asset source target)
   asset?