[Halla12_software] GEMC installation instruction and quick start guide

[Zhiwen Zhao]

hi, All

I have updated the installation instruction and added a quick start 
guide to wiki
https://hallaweb.jlab.org/wiki/index.php/Solid_sim_geant4#Quick_start
https://hallaweb.jlab.org/wiki/index.php/Solid_install#GEMC

The current production version for SoLID is GEMC 1.8 and Geant4.9.6.p01

We are testing GEMC 2.0 and evaluating the impact of the big version 
change. It will take some time.

Let me know any question or comment.

Thanks

Zhiwen