Documented line blocks in README.

README

@@ -1100,6 +1100,35 @@ This is equivalent to:
 To prevent all highlighting, use the `--no-highlight` flag.
 To set the highlighting style, use `--highlight-style`.
 
+Line blocks
+-----------
+
+**Extension: `line_blocks`**
+
+A line block is a sequence of lines beginning with a vertical bar (`|`)
+followed by a space.  The division into lines will be preserved in
+the output, as will any leading spaces; otherwise, the lines will
+be formatted as markdown.  This is useful for verse and addresses:
+
+    | The limerick packs laughs anatomical
+    | In space that is quite economical.
+    |    But the good ones I've seen
+    |    So seldom are clean
+    | And the clean ones so seldom are comical
+
+    | 200 Main St.
+    | Berkeley, CA 94718
+
+The lines can be hard-wrapped if needed, but the continuation
+line must begin with a space.
+
+    | The Right Honorable Most Venerable and Righteous Samuel L.
+      Constable, Jr.
+    | 200 Main St.
+    | Berkeley, CA 94718
+
+This syntax is borrowed from [reStructuredText].
+
 Lists
 -----