Set up continuous integration to build project on main branch

The continuous integration should run when (and only when) there is a new commit into the `main` branch.

It should build the project using **g++** to compile `Dice.cpp` with the options `-Wall -Werror` to report warnings as errors. It should build but not run the project.

With the build CI set up, its status badge should be added to the `README.md` file