Document requests that remixing and copying make

[towerofnix]

I think they use the same endpoint as normally saving a project does, but with the extra properties original_id (self-explanatory) and is_copy/is_remix (= 1).

See LLK/scratch-gui#3493 for reference.

Edit: Per the related PR LLK/scratch-storage#54, it looks like the entire endpoint is POST https://projects.scratch.mit.edu/?original_id=<id>&is_(copy, remix)=1 (whereas just uploading a new project without any original ID and not a copy/remix is POST https://projects.scratch.mit.edu/, and updating is PUT https://projects.scratch.mit.edu/<id>).

See also #37.