Add a "Contributing" section to README with patch submission guidelines For the CubicWeb project and its dependencies, we now prefer patches submission and review by email on a public mailing list. We are thus moving away from the previous vcreview-based workflow taking place on the forge. This change is motivated by the following points: - the current reviewer assignment mechanism (pick a random reviewer, rely on reviewer availability rather than on willingness to review, send related patches to distinct people, etc.) is inefficient if not counter-productive; - most of the times, discussion only happens between the patch submitter and a reviewer with no easy way to increase the audience; - cubicweb-vcreview has no concept of patch series; - cubicweb-vcreview is not actively maintained anymore and its usability keeps deteriorating. We expect that email-based submission and review of patches will circumvent these limitations. Anybody interested in the project is welcome to subscribed to the mailing list and participate to the review process. This patch documents the basic workflow of patches submissions by email.

[cwctl] do only clean static data dir content (closes #17069762) deleting the directory itself is useless and may be a problem in automated deployment environments (user may not have permissions to delete or create that directory).