docs: update language on commit messages & notability

[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?

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Updating the language to explain that our commit style is a requirement, instead of a preference. This will allow us to reduce the amount of autosquashed PR's.

Also putting an example for our notability check, which I haven't found documented anywhere. It seems to only be in:

brew/Library/Homebrew/utils/shared_audits.rb

Lines 112 to 115 in 6e7e56f

	if (metadata["forks_count"] < 30) && (metadata["subscribers_count"] < 30) &&
	(metadata["stargazers_count"] < 75)
	return "GitHub repository not notable enough (<30 forks, <30 watchers and <75 stars)"
	end

[MikeMcQuaid]

Seems fine.

Why is autosquashing undesirable?

[p-linnane]

It dispatches a replacement PR with a single squashed commit.

[ZhongRuoyu]

Autosquashing is also less effective in guiding people to do it right next time, IMO.