Add human readable documentation to speed profiles

[rajesvariparasa]

While the code (https://github.com/Project-OSRM/osrm-backend/blob/master/profiles/car.lua) gives a sense of different parameters that have been considered for modelling the speed profiles, it would be helpful to have an accompanying descriptive text in the readme or in-line comments in code on the different parameters that affect the speed -eg turns, type of highway, presence of traffic light, if any, so on.

Thank you!