Update docs to clarify common workflow regarding npm link of forked repo

I'm wondering if there's a simpler way to achieve some of the workflow advantages mentioned in https://github.com/ds300/patch-package/issues/15

Normally, when I create a PR for a dependency, I use npm link to test out the fix.

I had a fixed library still linked and I wondered if creating a patch from my changes would "just work" but the changes were not in the patch.

I can create a git patch file from my fix commit but then in order to apply it inside the node_modules, I'd guess that I'd have to edit every path mentioned in it. What if this was done programatically with a flag `--from-fork=../my-forked-repo`?