This PR adds instructions on setting up Git to use
[`sphobjinv`](https://github.com/bskinn/sphobjinv) to display diffs for
`objects.inv`.
Since `objects.inv` is compressed, we can't review changes through `git
diff`. Git _does_ tell you if the file has changed, but this isn't that
helpful as the compressed file can be different even if the uncompressed
contents are the same.
Git will default back to binary diff if it doesn't find a rule in
`.gitconfig` so adding `.gitattributes` shouldn't disrupt contributors
that don't follow the guide.
Hopefully this information will help keep more eyes on the file and spot
any unintended changes.
Here's an example of it in action (downgrading qiskit to 0.46):
<img width="950" alt="Screenshot 2024-03-05 at 14 20 04"
src="https://github.com/Qiskit/documentation/assets/36071638/5aabaca4-d175-4871-84c5-e583069e0c1a">
Frank Harkins