readme: update the contribution process
The inability to easily pull from pull request on bitbucket and the lack of
obsolescence support there are too much pain. Patchbombed contribution are
flowing so we drop the bitbucket mention in the README.
This software was downloaded fromhttps://bitbucket.org/marmoute/mutable-historyCopyright 2011 Peter Arrenbrecht <peter.arrenbrecht@gmail.com> Logilab SA <contact@logilab.fr> Pierre-Yves David <pierre-yves.david@ens-lyon.org> Patrick Mezard <patrick@mezard.eu>This software may be used and distributed according to the terms of the GNUGeneral Public License version 2 or any later version.On Debian systems, the complete text of the GNU General Public License version2 can be found in `/usr/share/common-licenses/GPL-2'.