docs(README): document suggestion limitations

[DanielWTQ]

Spent a while debugging only to find out that this was a part of the code of the action rather than something wrong on our end.

[DanielWTQ]

Limitation comes from

suggest-changes/index.js

Lines 24 to 34 in e512b94

	const pullRequestFiles = (
	await octokit.pulls.listFiles({ owner, repo, pull_number })
	).data.map((file) => file.filename)
	// Get the diff between the head branch and the base branch (limit to the files in the pull request)
	const diff = await getExecOutput(
	'git',
	['diff', '--unified=1', '--', ...pullRequestFiles],
	{ silent: true }
	)

[parkerbxyz]

Thanks for opening this pull request and for your patience!

This is actually a GitHub limitation, not necessarily a limitation of this action: https://github.com/orgs/community/discussions/9099.

I'm open to including a note about this limitation in the README. I'd probably like to include a note that review comments can only be left on/around edited lines: https://github.com/orgs/community/discussions/4452.