Update contributing docs with npx changeset step

[cheshire137]

What are you trying to accomplish?

This updates the documentation for contributors so that they're aware of npx changeset, hopefully so maintainers won't have to run this for them.

What approach did you choose and why?

Jon had to do it for me in #1985. Thought adding docs for it would cut that down!

What should reviewers focus on?

Meta, but should this change have a changeset entry??

Are additional changes needed?

• No, this PR should be ok to ship as is. 🚢

[changeset-bot]

⚠️ No Changeset found

Latest commit: 7f68614

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[jonrohan]

nice