[ADD] add `project_assignees` add-on

Add a field in ``project.task`` to assign users to task. This
completes ``user_id`` field, that is then considered as the owner of
the task. Assignment becomes optional and can now target multiple
users.

Task: JOI-13
Signed-off-by: Valentin Lab <valentin.lab@kalysto.org>

project_assignees/README.rst

@@ -0,0 +1,45 @@
+=================
+project_assignees
+=================
+
+Add multiple assignees field to project task
+
+Installation
+============
+
+Use Odoo normal procedure to install add-ons to install
+``project_assignees``.
+
+Known issues / Roadmap
+======================
+
+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
+=======
+
+Images
+------
+* Elabore: `Icon <https://elabore.coop/web/image/res.company/1/logo?unique=f3db262>`_.
+
+Contributors
+------------
+* Stéphan Sainléger <https://github.com/stephansainleger>
+
+Funders
+-------
+The development of this module has been financially supported by:
+* Elabore (https://elabore.coop)
+
+
+Maintainer
+----------
+
+This module is maintained by Elabore.