Building the Library updated

[Himujjal]

I have written a well documented way to compile this library in a G++ compiler for now. The BUILDING.md file will help new Users to easily compile this library in their systems.

Please review the file in doc/BUILDING.md.

Plus this will be my first Pull Request ever in my GitHub history. 🥇

[keryell]

Thank you for the contribution.
I think it would be nice to integrate this somehow into https://github.com/triSYCL/triSYCL/blob/master/doc/testing.rst and https://github.com/triSYCL/triSYCL/blob/master/doc/cmake.rst
By the way we do not use .md files in this project. Only ReST .rst
Could you make a ReST version of your work?
Thanks!