Add discussion of custom styles to MANUAL.txt.

MANUAL.txt

@@ -3827,6 +3827,54 @@ To disable highlighting, use the `--no-highlight` option.
 
 [highlighting-kate]: https://github.com/jgm/highlighting-kate
 
+Custom Styles in Docx Output
+============================
+
+By default, pandoc's docx output applies a predefined set of styles for
+blocks such as paragraphs and block quotes, and uses largely default
+formatting (italics, bold) for inlines. This will work for most
+purposes, especially alongside a `reference.docx` file. However, if you
+need to apply your own styles to blocks, or match a preexisting set of
+styles, pandoc allows you to define custom styles for blocks and text
+using `div`s and `span`s, respecitively.
+
+If you define a `div` or `span` with the attribute `custom-style`,
+pandoc will apply your specified style to the contained elements. So,
+for example,
+
+    <span custom-style="Emphatically">Get out,</span> he said.
+
+would produce a docx file with "Get out," styled with character
+style `Emphatically`. Similarly,
+
+    Dickinson starts the poem simply:
+
+    <div custom-style="Poetry">
+	| A Bird came down the Walk---
+	| He did not know I saw---
+	</div>
+	
+Would style the two contained lines with the `Poetry` paragraph style.
+
+If the styles are not yet in your reference.docx, they will be defined
+in the output file as inheriting from normal text. If they are already
+defined, pandoc will not alter the definition.
+
+This feature allows for greatest customization in conjunction with
+[pandoc filters]. If you want all paragraphs after block quotes to be
+indented, you can write a filter to apply the styles necessary. If you
+want all italics to be transformed to the `Emphasis` character style
+(perhaps to change their color), you can write a filter which will
+transform all italicized inlines
+
+    Emph <inlines>
+
+to 
+
+    Span ("", [], [("custom-style", "Emphasis")] <inlines>
+	
+[pandoc filters]: http://pandoc.org/scripting.html
+
 Custom writers
 ==============