[AIRFLOW-XXX] Add history become ASF top level project (#4757)

docs/project.rst

@@ -25,7 +25,10 @@ Airflow was started in October 2014 by Maxime Beauchemin at Airbnb.
 It was open source from the very first commit and officially brought under
 the Airbnb GitHub and announced in June 2015.
 
-The project joined the Apache Software Foundation's incubation program in March 2016.
+The project joined the Apache Software Foundation's Incubator program in March 2016 and the
+Foundation `announced Apache Airflow as a Top-Level Project
+<https://blogs.apache.org/foundation/entry/the-apache-software-foundation-announces44>`_
+in January 2019.
 
 
 Committers