Document how to use commandline tools to modify multiple folders

[oxinabox]

We should have instructions on how to work with the near duplicate code on multiple folders that can replace my old git cherrypick way

I believe it would go something like this:

The folder structure on this repo is a bit atypical, this is due to it depending a lot on julia Pkg internals.
So there is a folder for roughly each minor release of julia, with different (but often very similar) code in each.

The preferred way of making a PR for a new feature is:
Firstly, make the change in the latest version, git commit that and get a code review
Then once things are settled, use git diff origin/main > out.patch to save your changes into a patch file
Then for each version folder you need to move it to use patch -d src/$VERSION -p2 out.patch to apply the patch.
This won't always work but when it does it can make things much simpler than applying the changes manually.
Finally push another commit and request to be merged

I haven't tested this so I don't want to just add it.