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

spektom/incubator-airflow,danielvdende/incubator-airflow,wolfier/incubator-airflow,malmiron/incubator-airflow,spektom/incubator-airflow,cfei18/incubator-airflow,akosel/incubator-airflow,mtagle/airflow,mrkm4ntr/incubator-airflow,wooga/airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,mrkm4ntr/incubator-airflow,cfei18/incubator-airflow,apache/airflow,akosel/incubator-airflow,bolkedebruin/airflow,criccomini/airflow,r39132/airflow,adamhaney/airflow,yk5/incubator-airflow,apache/airflow,wileeam/airflow,yk5/incubator-airflow,subodhchhabra/airflow,DinoCow/airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,lxneng/incubator-airflow,mistercrunch/airflow,wolfier/incubator-airflow,subodhchhabra/airflow,spektom/incubator-airflow,CloverHealth/airflow,adamhaney/airflow,Acehaidrey/incubator-airflow,edgarRd/incubator-airflow,bolkedebruin/airflow,fenglu-g/incubator-airflow,yk5/incubator-airflow,mtagle/airflow,danielvdende/incubator-airflow,andyxhadji/incubator-airflow,criccomini/airflow,sekikn/incubator-airflow,edgarRd/incubator-airflow,artwr/airflow,akosel/incubator-airflow,wolfier/incubator-airflow,sergiohgz/incubator-airflow,lxneng/incubator-airflow,wileeam/airflow,dhuang/incubator-airflow,artwr/airflow,cfei18/incubator-airflow,r39132/airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,sekikn/incubator-airflow,Acehaidrey/incubator-airflow,apache/airflow,fenglu-g/incubator-airflow,mtagle/airflow,mtagle/airflow,malmiron/incubator-airflow,gtoonstra/airflow,r39132/airflow,Fokko/incubator-airflow,andyxhadji/incubator-airflow,DinoCow/airflow,airbnb/airflow,nathanielvarona/airflow,mrkm4ntr/incubator-airflow,bolkedebruin/airflow,lyft/incubator-airflow,dhuang/incubator-airflow,subodhchhabra/airflow,jgao54/airflow,airbnb/airflow,sekikn/incubator-airflow,artwr/airflow,sid88in/incubator-airflow,artwr/airflow,apache/incubator-airflow,lyft/incubator-airflow,mistercrunch/airflow,r39132/airflow,owlabs/incubator-airflow,yk5/incubator-airflow,nathanielvarona/airflow,nathanielvarona/airflow,cfei18/incubator-airflow,bolkedebruin/airflow,apache/airflow,owlabs/incubator-airflow,mistercrunch/airflow,airbnb/airflow,criccomini/airflow,DinoCow/airflow,Acehaidrey/incubator-airflow,malmiron/incubator-airflow,sergiohgz/incubator-airflow,criccomini/airflow,bolkedebruin/airflow,DinoCow/airflow,Fokko/incubator-airflow,akosel/incubator-airflow,wooga/airflow,subodhchhabra/airflow,fenglu-g/incubator-airflow,fenglu-g/incubator-airflow,wileeam/airflow,CloverHealth/airflow,CloverHealth/airflow,apache/incubator-airflow,mrkm4ntr/incubator-airflow,sid88in/incubator-airflow,sergiohgz/incubator-airflow,gtoonstra/airflow,Fokko/incubator-airflow,nathanielvarona/airflow,cfei18/incubator-airflow,dhuang/incubator-airflow,lxneng/incubator-airflow,wooga/airflow,sekikn/incubator-airflow,spektom/incubator-airflow,edgarRd/incubator-airflow,jgao54/airflow,wileeam/airflow,sid88in/incubator-airflow,dhuang/incubator-airflow,andyxhadji/incubator-airflow,sergiohgz/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,sid88in/incubator-airflow,malmiron/incubator-airflow,apache/airflow,CloverHealth/airflow,mistercrunch/airflow,lyft/incubator-airflow,gtoonstra/airflow,apache/airflow,lxneng/incubator-airflow,edgarRd/incubator-airflow,adamhaney/airflow,wooga/airflow,owlabs/incubator-airflow,jgao54/airflow,adamhaney/airflow,Fokko/incubator-airflow,apache/incubator-airflow,nathanielvarona/airflow,owlabs/incubator-airflow,danielvdende/incubator-airflow,apache/incubator-airflow,gtoonstra/airflow,andyxhadji/incubator-airflow,wolfier/incubator-airflow,airbnb/airflow,lyft/incubator-airflow,jgao54/airflow