Docs: Updating package readme with project composer package requirement for use in new projects

[coder-karen]

Proposed changes:

• This PR updates the package README.md to add additional info on requiring a package in a project's composer.json file, plus updating project versions and rebuilding, in order to be able to then use that package in another project.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

p1715621684054849/1715618744.314119-slack-C05PV073SG3

Does this pull request change what data or activity we track or use?

Testing instructions:

• Proof-read - the info is based on this Slack thread: p1715621684054849/1715618744.314119-slack-C05PV073SG3

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

• ✅ Include a description of your PR changes.
  
• ✅ Add a "[Status]" label (In Progress, Needs Team Review, ...).
  
• ✅ Add testing instructions.
  
• ✅ Specify whether this PR includes any changes to data or privacy.
  
• ✅ Add changelog entries to affected projects
  

This comment will be updated as you work on your PR and make changes. If you think that some of those checks are not needed for your PR, please explain why you think so. Thanks for cooperation 🤖

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