Docs: Added a pointer to the formatted docs to README

Kernel.org nicely maintains an online copy of the formatted documentation;
tell our users that it exists.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>

README

@@ -6,7 +6,9 @@ be rendered in a number of formats, like HTML and PDF. Please read
 Documentation/admin-guide/README.rst first.
 
 In order to build the documentation, use ``make htmldocs`` or
-``make pdfdocs``.
+``make pdfdocs``.  The formatted documentation can also be read online at:
+
+    https://www.kernel.org/doc/html/latest/
 
 There are various text files in the Documentation/ subdirectory,
 several of them using the Restructured Text markup notation.