Add migration-related tips to the "Merge Request Guidelines" doc

Why was this MR needed?

To avoid issues like #20606 (closed) cropping up again.

Added 1 commit:

• e18f27be - Add migration-related tips to the "Merge Request Guidelines" doc

Added 1 commit:

• c7864089 - Add migration-related tips to the "Merge Request Guidelines" doc

@axil: Could you please merge this in?

/cc @DouweM @rspeicher @rymai @stanhu @alfredo1 @rdavila

Reassigned to @axil

@timothyandrew might be worth to mention our migration guide (development/migration_style_guide.md)?

@axil For what it's worth, we already have a link to migrations style guide in the "Style guides" section: https://gitlab.com/gitlab-org/gitlab-ce/blob/c78640890cae5ca6f17e75c0e034503ff0de7cc9/CONTRIBUTING.md#style-guides

Milestone changed to %8.11

@rymai hmm true. Ok I'll merge this and then we can refactor as @sytses mentioned in https://gitlab.com/gitlab-org/gitlab-ce/commit/1de019f5c8a76da10877b8a2b4f27823374c6da2#note_13571415.

Enabled an automatic merge when the build for c7864089 succeeds

Status changed to merged

mentioned in commit f2efd892