Add link to marketplace

[Soheab]

Hyperlinks the header in the README to the install page of the extension.
This saves the user approximately 1.5secs to search it up themselves.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
  • I have updated the changelog with a quick recap of my changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)
• I have read and agree to the Developer Certificate of Origin for this contribution

[EvieePy]

You need to check the I have read and agree to the [Developer Certificate of Origin](https://developercertificate.org/) for this contribution before I can merge

[Soheab]

You need to check the I have read and agree to the [Developer Certificate of Origin](https://developercertificate.org/) for this contribution before I can merge

Done!

[EvieePy]

Thanks 👍