Add documentation to the Tracking package

[leogermani]

Adds a README file to the Tracking package along with documentation on how to use it.

Fixes #15929
Fixes #17569

Changes proposed in this Pull Request:

• Adds a README file to the Tracking package along with documentation on how to use it.

Jetpack product discussion

n/a

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

No

Testing instructions:

Proofread the documentation

• Ajax and PHP tracking were recently tested by me
• I tested pixel tracking a long time ago. It should still be working the same way
• Please confirm that the console command is the right one

[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.
  
• ✅ All commits were linted before commit.
  
• ✅ 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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Then, add the "[Status] Needs Team review" label and ask someone from your team review the code.
Once you’ve done so, switch to the "[Status] Needs Review" label; someone from Jetpack Crew will then review this PR and merge it to be included in the next Jetpack release.

changes applied