Added RetinaNet paper and a few empty lines

[dingran]

• Added the RetinaNet paper (https://arxiv.org/abs/1708.02002) in the new paper worth reading section. It was submitted in Aug 2017 already have 17 citations, the architecture showed substantial improvements over F-RNN and SSD
• Added a few separating empty lines between section head and lists (it is more compatible with other markdown formats to do so)