Set up CI for automated build

We need to set up continuous integration for the GitHub repo so that:

*  any time a pull request is made for the `main` branch, we make sure that the application **builds successfully** with the following compilation command:

```
g++ main.cpp --std=c++17 -o MyFave
```

* add a status badge to `README.md` that indicates the current status of the build on the `main` branch