[Feature Specification] Document expectations between Pending Review and Approval stages

[Alvaro Uria]

The feature-specification.rst[1] file describes the process to follow when creating new specs (and when a new spec should be created). As part of the process, the mentioned way to gather feedback is via the openstack-discuss mailing list.

I would like to suggest extending the document details around coordination or expectations when a spec is put on "Pending review" state, and the path towards having it approved.

* A possible information could be that the "Specs reviewers" go through pending approvals every N weeks/months.
* Another option is making notifications from spec authors via openstack-discuss (or any other means) mandatory.

On the other hand, specs require software engineer(s) assigned to it. Would this mean that the person writing the spec would need to work on the implementation as well (or at least the spec author(s) should coordinate with the people that would implement it)?

1. https://opendev.org/openstack/charm-guide/src/commit/0898ec5c755103d8bba76a247b87bd86f1c65753/doc/source/feature-specification.rst