Implement changes proposed for REP 2004 in one exemplary ROS 2 core package

[norro]

Implement changes proposed for REP 2004 (see #12) regarding documentation of requirements in one exemplary ROS 2 core package, e.g. rmw due to its nature as an interface library and the potential interest of ADLINK.

[robbel]

@norro a quick note that in today's safety working group we discussed @togaen's work to add requirements in our fork of Autoware's ground classifier. We are happy to share lessons learned either here or in the WG meeting.