Update link to NPM package guidelines in readme

[rgaban]

• Issues #508: NPM package guidelines link has moved
• Updated link to npm package guidelines link in readme

Author Checklist

• [ ] Add unit test(s)
• [x] Update version in package.json (see the versioning guidelines)
• [x] Update documentation (if necessary)
• [ ] Add story to storybook (if necessary)
• [x] Assign dev reviewer

[chawes13]

@rgaban I have exciting news! We were able to change the default branch to main, so the appropriate link is now: https://github.com/LaunchPadLab/opex-public/blob/main/gists/npm-package-guidelines.md