Improve feedback message after installing or upgrading airflow with helm chart (#15820)
Improve feedback message after installing or upgrading airflow with helm chart (#15820)

With the aid of Helm chart's `templates/NOTES.txt` file, some nifty messages and/or instructions can be printed out to STDOUT for users after they run either the `helm install` or `helm upgrade` command.

This PR aims to improve the user experience when using Helm chart to install Airflow (or upgrade Airflow) in a kubernetes cluster. It contains handy "next steps" and essential information about a fresh Airflow installation or upgrade.

closes #15315 
apache/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,dhuang/incubator-airflow,apache/airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,mistercrunch/airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,dhuang/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,apache/incubator-airflow,mistercrunch/airflow,lyft/incubator-airflow,nathanielvarona/airflow,lyft/incubator-airflow,nathanielvarona/airflow,lyft/incubator-airflow,bolkedebruin/airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,sekikn/incubator-airflow,cfei18/incubator-airflow,bolkedebruin/airflow,apache/incubator-airflow,cfei18/incubator-airflow,apache/airflow,danielvdende/incubator-airflow,sekikn/incubator-airflow,Acehaidrey/incubator-airflow,mistercrunch/airflow,nathanielvarona/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,sekikn/incubator-airflow,dhuang/incubator-airflow,sekikn/incubator-airflow,apache/incubator-airflow,apache/incubator-airflow,nathanielvarona/airflow,bolkedebruin/airflow,dhuang/incubator-airflow,Acehaidrey/incubator-airflow,mistercrunch/airflow,apache/airflow,lyft/incubator-airflow,apache/airflow