Add airflow task lifecycle documentation (MarquezProject#870)

Signed-off-by: henneberger <[email protected]>

Co-authored-by: Willy Lulciuc <[email protected]>

integrations/airflow/docs/task-lifecycle.md

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+# Airflow task lifecycle
+
+This document describes the airflow task lifecycle and describes how marquez hooks into tasks to obtain information.
+
+## Airflow lifecycle overview
+https://airflow.apache.org/docs/apache-airflow/stable/concepts.html#task-lifecycle
+
+![](task_lifecycle_diagram.png)
+
+The happy flow consists of the following stages:
+1. No status (scheduler created empty task instance, a job and run with nominal start/end time is reported to marquez)
+2. Scheduled (scheduler determined task instance needs to run)
+3. Queued (scheduler sent task to executor to run on the queue)
+4. Running (worker picked up a task and is now running it)
+5. Success/Skipped/Failed (task completed, marquez is notified of the task's actual start/stop time with the status of the task instance)
+
+## Marquez Interaction
+The Marquez airflow integration is only called from the scheduler instance. The scheduler periodically polls task instances to check the states of the current tasks. When the scheduler finds a state transitions in a `success`, `skipped`, or `failed` state, it notifies the dag via a `handle_callback` function. Since marquez relies on the scheduler to be notified of a state transition, there may be a delay from when a task transitions state to when it is reported to marquez.