[polaris.shopify.com] Add new content section to icon contribution

[heyjoethomas]

We decided to create a new workflow for people who need to contribute a new icon but aren't comfortable submitting a pull request. This new section details that process and dovetails with the new issue templates recently created in the Polaris repo.

Before

After

How to 🎩

🖥 Local development instructions
🗒 General tophatting guidelines
📄 Changelog guidelines

Copy-paste this code in playground/Playground.tsx:

import React from 'react';
import {Page} from '../src';

export function Playground() {
  return (
    <Page title="Playground">
      {/* Add the code you want to test in here */}
    </Page>
  );
}

🎩 checklist

• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility
• Updated the component's README.md with documentation changes
• Tophatted documentation changes in the style guide

[zecarlostorre]

Thanks for pushing this so quickly!