Signed-off-by: Rémy Coutable <remy@rymai.me>

After the 7th it is OK to merge code changes that are merged as long it
is behind a feature flag.

This adds a development guide explaining that we are going to use
feature flags more often, why, what the benefits are, and so on.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/49619 for more
information.

- Bugs are the parent section
- Use the same workflow but branch off bugs / regression sections
- Addressed review comments

Regressions that can bypass the exception requests are the ones which `regression:xx.x` is the last recent monthly release or the current release.

The example provided was broken because it mistakes
a bug for a regression. It is not needed for clarity
anyway. The milestone part is no longer relevant as
well.

Signed-off-by: Rémy Coutable <remy@rymai.me>