Automate build workflow

We need to add a CI workflow that tries to build `main.cpp` using the g++ compiler with argument `--std=c++17`. We should add the status badge of the workflow to `README.md` so we can see whether or not our latest push to the main branch is building successfully.

Start with main.cpp and README.md, then review the repository's existing configuration to determine where a CI workflow belongs. Verify that the workflow builds main.cpp with g++ and --std=c++17, and that README.md displays its status badge for pushes to the main branch.

Written by the indexing model from the issue text.