Document availability of CMake build system

CMake is available as build system for `dftd4` and should be documented.

The standard workflow for building with CMake should be sufficient (configure, build, test, install):

```
cmake -B _build -G Ninja -DCMAKE_INSTALL_PREFIX=$HOME/.local
cmake --build _build
ctest --test-dir build --parallel --output-on-failure
cmake --install _build
```