Add information about simsycl headers to README

[Pennycook]

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

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 <sycl/sycl.hpp>. I had to poke around in the source to figure out that I needed to include simsycl/system.hh as well.