Add robots.txt

I had to rollback the line removed by @eduardofiorezi on #148 - https://github.com/Helabs/pah/pull/148/files#diff-195a32a00ea5ea5336f1e8f188508551L8

This way we can have a **robots.txt** with our default settings, as suggested by @mergulhao on #152 

Fix #152