Added more explanation of metadata in README.

README

@@ -1843,6 +1843,23 @@ equivalent of the markdown in the `abstract` field:
     <p>This is the abstract.</p>
     <p>It consists of two paragraphs.</p>
 
+Note:  The example above will not work with the default templates.
+The `author` variable in the templates expects a simple list or string,
+and there is no `abstract` variable in most templates.  To use these,
+you would need to use a custom template with appropriate variables.
+For example:
+
+    $for(author)$
+    $if(author.name)$
+    $author.name$$if(author.affiliation)$ ($author.affiliation$)$endif$
+    $else$
+    $author$
+    $endif$
+    $endfor$
+
+    $if(abstract)$
+    Abstract: $abstract$
+    $endif$
 
 Backslash escapes
 -----------------