Add environment variable setup instructions to README.md

[Piumal1999]

Is your feature request related to a problem? Please describe.
We removed the application.yml.example file recently. So the developers can directly use t3 application.yml file. But we should prevent them from committing a changed application.yml file.

Describe the solution you'd like
It's better if we can provide the instructions to setup the environment variables instead of editing the application.yml file. Do some research and update the readme file with the suitable instructions.
Also include a checkbox in pr template to ensure that the developer hasn't committed the application.yml file.

Describe alternatives you've considered
N/A

Additional context
N/A

[YoshithaRathnayake]

Will I able to do this @Piumal1999 ?

[pasindur99]

Note: mail credentials are in the simplejavamail.properties file. Note it as well, in the guidelines.