README: Improve documentation descriptions

"This file" indeed was moved once, but at some point "this file", the
top-level README, becomes a file in itself. Now that time has come :)

Let's describe how things are, and suggest reading "this file" first,
"this file" simply being a the admin-guide README file, not a file that
was once moved.

Signed-off-by: Martin Kepplinger <martink@posteo.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

README

 Linux kernel
 ============
 
-This file was moved to Documentation/admin-guide/README.rst
-
-Please notice that there are several guides for kernel developers and users.
-These guides can be rendered in a number of formats, like HTML and PDF.
+There are several guides for kernel developers and users. These guides can
+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``.