Update README to describe new release process

[tjmw]

What does this change?

Updates the docs to describe the process of releasing to NPM. We've migrated to changesets and releasing from CI (see also #435 and #436).

How to test

How can we measure success?

Have we considered potential risks?

Images

Accessibility

• Tested with screen reader
• Navigable with keyboard
• Colour contrast passed
• The change doesn't use only colour to convey meaning

[changeset-bot]

⚠️ No Changeset found

Latest commit: a324c63

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.

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