Add information about simsycl headers to README

The README currently says:

> Systems are defined by listing all devices, platforms and their runtime properties in a JSON file.
> As a starting point, you can export the built-in system definition via
> ```c++
> simsycl::write_system_config("system.json", simsycl::builtin_system);
> ```

Trying to follow these instructions gives a compiler error, because the `simsycl::` functions aren't pulled in by including ``. I had to poke around in the source to figure out that I needed to include `simsycl/system.hh` as well.