doc: add maintainer book

There is currently very little documentation in the kernel on maintainer
level tasks. In particular there are no documents on creating pull
requests to submit to Linus.

Quoting Greg Kroah-Hartman on LKML:

    Anyway, this actually came up at the kernel summit / maintainer
    meeting a few weeks ago, in that "how do I make a
    good pull request to Linus" is something we need to document.

    Here's what I do, and it seems to work well, so maybe we should turn
    it into the start of the documentation for how to do it.

(quote references: kernel summit, Europe 2017)

Create a new kernel documentation book 'how to be a maintainer'
(suggested by Jonathan Corbet). Add chapters on 'configuring git' and
'creating a pull request'.

Most of the content was written by Linus Torvalds and Greg Kroah-Hartman
in discussion on LKML. This is stated at the start of one of the
chapters and the original email thread is referenced in
'pull-requests.rst'.

Signed-off-by: Tobin C. Harding <me@tobin.cc>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>