Document how it works

[niyue]

I feel interested about the project, but the current documentation is brief on how poac works, and I would like to understand how it works:

• what is possible if using poac, especially about
  • specifying compiler/linker arguments
  • integrating with existing CMake based projects
• what cannot be done if using poac

Thanks.

[ken-matsui]

These are now well documented