[doc] Document usage of changelog

At the moment project documentation does not contain any mention
on how changelog should be produced.

Some use reno, some use fragments. At the moment there is no transition
from reno to antsibull format is available, so let's ask contributors to
follow ansible's format of changelog fragments.

In case we come up with a transition layer from reno to fragment format,
or antsibull replacement, we can change documentation.

Change-Id: I048f80af849d2872be866e44882d2a8fcbf01cfc
Signed-off-by: Dmitriy Rabotyagov <dmitriy.rabotyagov@cleura.com>

docs/reviewing.md

@@ -22,6 +22,8 @@ How to do a review? What to look for when reviewing patches?
   attributes to `name` to be consistent with other modules and with openstacksdk. When refactoring a module, then add
   the old attribute as an alias to keep backward compatibility.
 * Does the module have integration tests in `ci/roles`?
+* Does the patch include a changelog fragment? Every new feature, or important bugfix must include one. Ensure that
+  the fragment follows the Ansible format and that `reno` is not used.
 * Is documentation in `DOCUMENTATION`, `RETURN` and `EXAMPLES` up to date?
 * Does `RETURN` list all values which are returned by the module?
 * Are descriptions, keys, names, types etc. in `RETURN` up to date and sorted?