Update README with current execution workflow with NEURON (#374)

* Update README with latest workflow of NEURON/CoreNEURON installation
* Cleanup and integration with existing README
* Add citation and contribution information
* Mention flex, bison dependencies from NEURON.
* Provide link to documentation for BBPCOREPOINTER

fixes #222  #190

Co-authored-by: Alexandru Savulescu <[email protected]>
Co-authored-by: Omar Awile <[email protected]>

Author: 3 people