README: document new image with caption feature.

An image in a paragraph by itself is treated as a figure,
with the alt text the caption.

git-svn-id: https://pandoc.googlecode.com/svn/trunk@1890 788f1e2b-df1e-0410-8736-df70ead52e1b

README

@@ -909,6 +909,23 @@ code block above will appear as follows:
       </code>
     </pre>
 
+Images with captions
+--------------------
+
+An image occurring by itself in a paragraph will be rendered as
+a figure with a caption.  (In LaTeX, a figure environment will be
+used; in HTML, the image will be placed in a `div` with class
+`figure`, together with a caption in a `div` with class `caption`.)
+The image's alt text will be used as the caption.
+
+    ![This is the caption](/url/of/image.png)
+
+If you just want a regular inline image, just make sure it is not
+the only thing in the paragraph. One way to do this is to insert a
+nonbreaking space after the image:
+
+    ![This image won't be a figure](/url/of/image.png)\ 
+
 Title blocks
 ------------