Document how to make changes from latest copy of upstream project / Document how to update fork with latest code from upstream
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### Problem to solve
If a user is looking to make a change to upstream project it's not clear how to do so using latest copy of upstream if user previously forked project.
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### Proposal
* [ ] Document how to update fork with latest code from upstream
* Using GitLab UI
* Some incomplete details at https://docs.gitlab.com/ee/user/project/repository/forking_workflow.html#repository-mirroring
* Mirroring mirrors all branch but they might just want latest copy of main branch
* How to do it using free version?
* Using Command Line
* https://stackoverflow.com/questions/7244321/how-do-i-update-or-sync-a-forked-repository-on-github
* https://forum.gitlab.com/t/refreshing-a-fork/32469/2
* [ ] Document how to create new branch from upstream repo w/ all current commits
* Selecting edit button from upstream repo and making change following workflow results in new branch created in your fork with latest commits
* Also works from web IDE?
* Work from create merge request button in issues?
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