Add Scarf Pixel to Readme (apache#39533)

Similar to apache/superset#25065 and apache#39510 but this one doesn't need to wait for the VOTE since this embed it to Readme. More detail in https://docs.scarf.sh/web-traffic/

All of this data will be available to interested PMC members, and reported periodically in things like Town Hall & newsletters.

README.md

@@ -532,3 +532,6 @@ The CI infrastructure for Apache Airflow has been sponsored by:
 
 <a href="https://astronomer.io"><img src="https://assets2.astronomer.io/logos/logoForLIGHTbackground.png" alt="astronomer.io" width="250px"></a>
 <a href="https://aws.amazon.com/opensource/"><img src="docs/integration-logos/aws/[email protected]" alt="AWS OpenSource" width="130px"></a>
+
+<!-- telemetry/analytics pixel: -->
+<img referrerpolicy="no-referrer-when-downgrade" src="https://static.scarf.sh/a.png?x-pxid=1b5a5e3c-da81-42f5-befa-42d836bf1b54" alt="Tracking Pixel" />