[AIRFLOW-2536] docs about how to deal with airflow initdb failure

Add docs to faq.rst to talk about how to deal with
Exception: Global variable
explicit_defaults_for_timestamp needs to be on (1)
for mysql

Closes #3429 from milton0825/fix-docs

docs/faq.rst

@@ -161,3 +161,11 @@ How can we reduce the airflow UI page load time?
 ------------------------------------------------
 
 If your dag takes long time to load, you could reduce the value of ``default_dag_run_display_number`` configuration in ``airflow.cfg`` to a smaller value. This configurable controls the number of dag run to show in UI with default value 25.
+
+How to fix Exception: Global variable explicit_defaults_for_timestamp needs to be on (1)?
+---------------------------------------------------------------------------------------------
+
+This means ``explicit_defaults_for_timestamp`` is disabled in your mysql server and you need to enable it by:
+
+#. Set ``explicit_defaults_for_timestamp = 1`` under the mysqld section in your my.cnf file.
+#. Restart the Mysql server.