readme: Tweak style
readme: Tweak style

This patch includes:
* make headers be surrounded by blank lines
* eliminate tailing spaces & consecutive blanks
* make lists be surrounded by blank lines
* fix typo
* bare URLs embrace angel brackets
* fix ordered list item prefix
* fix missing line breaks

Adlink-ROS/adlink_ddsbot,Adlink-ROS/adlink_ddsbot