[doc/book] Add a note about <database>-support virtual package.
Cubicweb setup documentation now mention cubicweb-postgresql-support and cubicweb-mysql-support
# copyright 2003-2010 LOGILAB S.A. (Paris, FRANCE), all rights reserved.# contact http://www.logilab.fr/ -- mailto:contact@logilab.fr## This file is part of CubicWeb.## CubicWeb is free software: you can redistribute it and/or modify it under the# terms of the GNU Lesser General Public License as published by the Free# Software Foundation, either version 2.1 of the License, or (at your option)# any later version.## CubicWeb is distributed in the hope that it will be useful, but WITHOUT# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS# FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more# details.## You should have received a copy of the GNU Lesser General Public License along# with CubicWeb. If not, see <http://www.gnu.org/licenses/>."""pre 3.6 bw compat"""# pylint: disable-msg=W0614,W0401fromwarningsimportwarnwarn('moved to cubicweb.mixins',DeprecationWarning,stacklevel=2)fromcubicweb.mixinsimport*