[ci skip] Specify rebase location in CONTRIBUTING

[hyperdefined]

As someone who doesn't understand how git works super well, I was confused when trying to rebase. Turns out I was in the wrong folder, and thought this could be added to help others understand where they need to rebase.

[lynxplay]

All of the commands in these sections are to be run in the respective -server or -api directories.
At best we could add something to the

Modifying Patches
Modifying previous patches is a bit more complex:

block saying that the below commands, unless specified otherwise, are ran in the respective -api or -server folders.

[Leguan16]

All of the commands in these sections are to be run in the respective -server or -api directories.

At best we could add something to the

Modifying Patches

Modifying previous patches is a bit more complex:

block saying that the below commands, unless specified otherwise, are ran in the respective -api or -server folders.

That what I wanted to propose. But then I thought, it's only confusing until you understand patches. And that each patch is a commit to the -API, -Server module, and when you understand that, I think it's clear that you need to run these commands in those directories.