Mention add_column_with_default in downtime guide

## What does this MR do?

This extends the "What causes downtime" guide with a mention of the `add_column_with_default` migration helper.

## Are there points in the code the reviewer needs to double check?

Spelling, grammar, etc.

## Why was this MR needed?

Currently the guide may lead one to believe it's not possible at all to add a column with a default value.

## Does this MR meet the acceptance criteria?

- [x] ~~[CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added~~
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- Tests
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5719