Document YAML metadata blocks.

README

@@ -733,7 +733,8 @@ will be replaced by the document title.
 To write a literal `$` in a template, use `$$`.
 
 Some variables are set automatically by pandoc.  These vary somewhat
-depending on the output format, but include:
+depending on the output format, but include metadata fields (such
+as `title`, `author`, and `date`) as well as the following:
 
 `header-includes`
 :   contents specified by `-H/--include-in-header` (may have multiple
@@ -748,13 +749,6 @@ depending on the output format, but include:
     multiple values)
 `body`
 :   body of document
-`title`
-:   title of document, as specified in title block
-`author`
-:   author of document, as specified in title block (may have
-    multiple values)
-`date`
-:   date of document, as specified in title block
 `lang`
 :   language code for HTML or LaTeX documents
 `slidy-url`
@@ -801,8 +795,8 @@ depending on the output format, but include:
 :   footer in man pages
 
 Variables may be set at the command line using the `-V/--variable`
-option. This allows users to include custom variables in their
-templates.
+option.  Variables set in this way override metadata fields with
+the same name.
 
 Templates may contain conditionals.  The syntax is as follows:
 
@@ -829,6 +823,11 @@ consecutive items:
 
     $for(author)$$author$$sep$, $endfor$
 
+A dot can be used to select a field of a variable that takes
+an object as its value.  So, for example:
+
+    $author.name$ ($author.affiliation$)
+
 If you use custom templates, you may need to revise them as pandoc
 changes.  We recommend tracking the changes in the default templates,
 and modifying your custom templates accordingly. An easy way to do this
@@ -1780,7 +1779,42 @@ YAML metadata block
 
 **Extension: `yaml_metadata_block`**
 
-TODO
+If the file begins with a YAML object, delimited by a line of three
+hyphens (`---`) at the top and a line of three hyphens (`---`) or three
+dots (`...`) at the bottom, metadata will be taken from the fields
+of the YAML object.  Metadata can contain lists and objects (nested
+arbitrarily), but all string scalars will be interpreted as markdown.
+
+Fields with names ending in an underscore will be ignored by
+pandoc.  (They may be given a role by external processors.)
+
+Note that YAML escaping rules must be followed. Thus, for example,
+if a title contains a colon, it must be quoted.  The pipe character
+(`|`) can be used to begin an indented block that will be interpreted
+literally, without need for escaping.  This form is necessary
+when the field contains blank lines:
+
+   ---
+   title:  'This is the title: it contains a colon'
+   author:
+   - name: Author One
+     affiliation: University of Somewhere
+   - name: Author Two
+     affiliation: University of Nowhere
+   tags: [nothing, nothingness]
+   abstract: |
+     This is the abstract.
+
+     It consists of two paragraphs.
+   ...
+
+Template variables will be set from the metadata.  Thus, for example,
+in writing HTML, the variable `abstract` will be set to the HTML
+equivalent of the markdown in the `abstract` field:
+
+    <p>This is the abstract.</p>
+    <p>It consists of two paragraphs.</p>
+
 
 Backslash escapes
 -----------------