Improve Readme

This is an attempt to improve the readme following a discussion in #182. My goal was to provide a clear
instructions to get started and accomplish the basic tasks with the toolkit source code. 

I also tried to document the pieces of the toolkit, places they can be found in the source code and how the build process assembles them (i.e. "what goes where"). Note that this captures the current state of affairs which will probably change when #180, #181 are resolved an eventually even more based on to the outcome of #177.

Finally, I extracted the release and versioning section into a separate file as it's not meant for the majority of toolkit users.

Hopefully this will make the project more accessible to newcomers.  (when inevitable factual issues are fixed that is :)).