Document how to add tests in contributing docs

[spenserblack]

This way, even if a user isn't able to fix an issue, they can still contribute.

Perhaps feedback.test.ts for community-driven tests. Feedback should probably be manually written, so no snapshot tests. Maybe something like this:

// feedback.test.ts
// NOTE Setting issueTitle to a number is a shortcut to making the title "Issue #123"
type FeedbackTest = [issueTitle: number | string, input: string, expected: string];

const feedback: FeedbackTest[] = [];

// NOTE If you're just contributing a test, you don't need to read past this line
const tests: [title: string, input: string, expected: string][] = feedback.map(([title, ...test]) => [typeof title === "number" ? `Issue #${title}` : title, ...test]);
test.each(tests)("%s", (_title, input, expected) => {
  expect(parse(input)).toBe(expected);
});