[ADD] project_timebox: create addon to estimate task effort

Adds fields to estimate the min and max time needed to solve a task.

Task: [JOI-13](https://justodooit.fr/mail/view?model=project.task&res_id=13&access_token=cca0b860-25d1-472d-9da1-76f68b01a932)

project_timebox/README.rst

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+===============
+project_timebox
+===============
+
+Add timebox field in tasks to estimate resolution effort.
+
+Installation
+============
+
+Use Odoo normal module installation procedure to install ``project_timebox``.
+To configure the timeboxes, go to Project > Configuration > Timeboxes
+
+Known issues / Roadmap
+======================
+
+None yet.
+
+Bug Tracker
+===========
+
+Bugs are tracked on `our issues website <https://github.com/elabore-coop/project-tools/issues>`_. In case of
+trouble, please check there if your issue has already been
+reported. If you spotted it first, help us smashing it by providing a
+detailed and welcomed feedback.
+
+Credits
+=======
+
+Contributors
+------------
+
+* Stéphan Sainléger (https://github.com/stephansainleger)
+* Nicolas Jeudy (https://github.com/njeudy)
+
+Funders
+-------
+
+The development of this module has been financially supported by:
+* Elabore (https://elabore.coop)
+* Mycéliandre (https://myceliandre.fr)
+* Lokavaluto (https://lokavaluto.fr)
+
+
+Maintainer
+----------
+
+This module is maintained by Elabore.