Add note about backports
What does this MR do?
updates the documentation to include a note about backports after i was unsure about which branch to target when opening a merge request.
What are the relevant issue numbers?
Does this MR meet the acceptance criteria?
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Changelog entry added  - 
Documentation created/updated for GitLab EE, if necessary  - 
Documentation created/updated for this project, if necessary  - 
Documentation reviewed by technical writer or follow-up review issue created  - 
Tests added for this feature/bug  - 
Job definition updated, if necessary  - 
Conforms to the code review guidelines  - 
Conforms to the Go guidelines  - 
Security reports checked/validated by reviewer