docs: Move files from docs/qmp/ to docs/

docs: Move files from docs/qmp/ to docs/

Giving QMP its own subdirectory in docs/ is hardly worthwhile when we
have just four files, and one of them isn't even in the subdirectory.
Move the files from docs/qmp/ to docs/, renaming docs/qmp/README to
docs/qmp-intro.

Update MAINTAINERS.  The new pattern also captures the fourth file
docs/writing-qmp-commands.txt.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1443111117-29831-2-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>