README: add a new README file, pointing to the Documentation/

As we moved the real README file to Documentation/admin-guide/README.rst,
let's add a replacement, pointing to it, and giving the main directions
about documentation.

In the future, perhaps it would be worth to move the contents
of Documentation/00-Index into this README.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

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.
+
+In order to build the documentation, use ``make htmldocs`` or
+``make pdfdocs``.
+
+There are various text files in the Documentation/ subdirectory,
+several of them using the Restructured Text markup notation.
+See Documentation/00-INDEX for a list of what is contained in each file.
+
+Please read the Documentation/process/changes.rst file, as it contains the
+requirements for building and running the kernel, and information about
+the problems which may result by upgrading your kernel.