Clarify documentation and mail server issue
I made small adjustments to the README.md file so now it has the steps of having the required files for the tests to pass. I also updated an old link that did not work anymore and made it more clear where test credentials can be found.
Initially the mail server error that occurs upon first startup was only included in the "Common Issues" section, I made the solution for that the default and changed the application.yml.template.