Add license details

While most of the content included in this template already has its own license (godot-cpp, godot_headers and godot_cpp_helper_plugin), I guess it would make sense to also have a license document covering the rest (config, buildsystem).

I'd suggest something like Public domain / Unlicense to be as permissive as possible (while attribution for your work should still be given as per the MIT license for godot_cpp_helper_plugin).