Add note about global .gitignore to CONTRIBUTING.md — Instead of ignoring .DS_Store files created by macOS Finder (#6499)

# Objective

Finder in macOS creates hidden `.DS_Store` files containing metadata (for icon positioning, view mode, etc) whenever you browse a directory. There's no point in committing these to git, and they're a common git + macOS nuisance.

## Solution

- ~~This PR adds `.DS_Store` files to `.gitignore`, improving the developer experience on macOS.~~
- This PR adds a note to the `CONTRIBUTING.md` file teaching how to use global git ignore.

CONTRIBUTING.md

@@ -282,6 +282,7 @@ If you're new to Bevy, here's the workflow we use:
 1. Fork the `bevyengine/bevy` repository on GitHub. You'll need to create a GitHub account if you don't have one already.
 2. Make your changes in a local clone of your fork, typically in its own new branch.
    1. Try to split your work into separate commits, each with a distinct purpose. Be particularly mindful of this when responding to reviews so it's easy to see what's changed.
+   2. Tip: [You can set up a global `.gitignore` file](https://docs.github.com/en/get-started/getting-started-with-git/ignoring-files#configuring-ignored-files-for-all-repositories-on-your-computer) to exclude your operating system/text editor's special/temporary files. (e.g. `.DS_Store`, `thumbs.db`, `*~`, `*.swp` or `*.swo`) This allows us to keep the `.gitignore` file in the repo uncluttered.
 3. To test CI validations locally, run the `cargo run -p ci` command. This will run most checks that happen in CI, but can take some time. You can also run sub-commands to iterate faster depending on what you're contributing:
     * `cargo run -p ci -- lints` - to run formatting and clippy
     * `cargo run -p ci -- test` - to run tests