docs: update commit style guidelines

[p-linnane]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

We ask people to check the commit style guidelines when submitting PR's, but we never updated them to state we prefer one squashed commit now. Otherwise the autosquash workflow dispatches replacement PR's, which wastes CI time.

[apainintheneck]

LGTM! It might be good to mention the why too but maybe that's too much information to include in the formula cookbook.

[MikeMcQuaid]

Thanks again @p-linnane!

[EricFromCanada]

Does the line "Do not squash commits after updating a pull request" from https://github.com/Homebrew/homebrew-cask/blob/master/CONTRIBUTING.md#style-guide still apply?

[p-linnane]

Does the line "Do not squash commits after updating a pull request" from https://github.com/Homebrew/homebrew-cask/blob/master/CONTRIBUTING.md#style-guide still apply?

This doesn't apply to homebrew-cask, just homebrew-core.