ADD: Note on how to setup dns hostname resolution to installation manual

Hi,

I'm suggesting to add the following note to the installation instructions. It took me some time to figure out why the test failed and why I wasn't able to push. The error messages did not point into the right direction so this might help other people with the same problems.

I hope my english is good enough, if not, please feel free to rephrase anything.

Thank you in advance,

Holger Segnitz