[AIRFLOW-3341] FAQ return DAG object example (#4605)
[AIRFLOW-3341] FAQ return DAG object example (#4605)

* added example of a function returning a dag object

apache/airflow,DinoCow/airflow,lyft/incubator-airflow,dhuang/incubator-airflow,mtagle/airflow,danielvdende/incubator-airflow,Fokko/incubator-airflow,wooga/airflow,DinoCow/airflow,mtagle/airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,bolkedebruin/airflow,apache/airflow,spektom/incubator-airflow,wooga/airflow,mrkm4ntr/incubator-airflow,r39132/airflow,nathanielvarona/airflow,nathanielvarona/airflow,DinoCow/airflow,mrkm4ntr/incubator-airflow,sekikn/incubator-airflow,Fokko/incubator-airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,wileeam/airflow,mistercrunch/airflow,bolkedebruin/airflow,wileeam/airflow,nathanielvarona/airflow,mtagle/airflow,spektom/incubator-airflow,wooga/airflow,apache/airflow,apache/airflow,cfei18/incubator-airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,airbnb/airflow,apache/incubator-airflow,wileeam/airflow,lyft/incubator-airflow,dhuang/incubator-airflow,Acehaidrey/incubator-airflow,r39132/airflow,nathanielvarona/airflow,wooga/airflow,danielvdende/incubator-airflow,Fokko/incubator-airflow,apache/incubator-airflow,r39132/airflow,apache/incubator-airflow,cfei18/incubator-airflow,airbnb/airflow,apache/incubator-airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,airbnb/airflow,mistercrunch/airflow,sekikn/incubator-airflow,dhuang/incubator-airflow,Acehaidrey/incubator-airflow,spektom/incubator-airflow,cfei18/incubator-airflow,wileeam/airflow,apache/airflow,DinoCow/airflow,cfei18/incubator-airflow,sekikn/incubator-airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,mistercrunch/airflow,dhuang/incubator-airflow,r39132/airflow,sekikn/incubator-airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,apache/airflow,nathanielvarona/airflow,spektom/incubator-airflow,airbnb/airflow,danielvdende/incubator-airflow,mtagle/airflow,mistercrunch/airflow,mrkm4ntr/incubator-airflow,Acehaidrey/incubator-airflow,mrkm4ntr/incubator-airflow,cfei18/incubator-airflow,Fokko/incubator-airflow