Contribution guidelines/maintainers: flow for editing/adding new files

[avivace]

<ISSOtm> Right, yes, you can't do that for files that haven't been edited. But, since you have upstream push access, you can push directly to the PR branch!
<ISSOtm> As long as you keep it as separated commits, then those can be reverted separately; it's also possible to restrict the changes being reviewed in the Files tab to specific commit (ranges), so it should be okay