Align contributing doc with oslo's policy
Align contributing doc with oslo's policy

Related to:
- https://review.opendev.org/#/c/723044/
- https://governance.openstack.org/tc/goals/selected/ussuri/project-ptl-and-contrib-docs.html

Change-Id: I35eb20fdf6470782109d9713ed61dff913caefd0

openstack/oslo.i18n