post: Document image `preview` and `caption` options

archetypes/post.md

@@ -10,8 +10,12 @@ categories = []
 
 # Featured image
 # Place your image in the `static/img/` folder and reference its filename below, e.g. `image = "example.jpg"`.
+# Use `caption` to display an image caption.
+#   Markdown linking is allowed, e.g. `caption = "[Image credit](http://example.org)"`.
+# Set `preview` to `false` to disable the thumbnail in listings.
 [header]
 image = ""
 caption = ""
+preview = true
 
 +++