add note on packages, pip and virtual envs. Add build tools header.

I've added a note in the readme.rst for the terminally naive (or stupid) like myself that if your linux is packaged based, and it doesn't have current packages that it is best to install them in the virtual env. Thus hopefully avoiding linux getting confused about which version (package or pip) to use.

I also added a Build Tools sub heading as it seemed useful.

I've put them in what seemed to me to be the best place.