Adding a guide on how to link HDL kernels

[JOOpdenhoevel]

This PR adds a guide on how to create HDL kernels for SLASH. It is basically a re-synthesis of UG1393, section "Packaging RTL kernels", since the requirements are also almost the same. However, I did not include a specific description with individual steps on how to build and integrate an HDL kernel since I would need to create a new example application for that. Since the issue #70 only called for documentation, I therefore did not go so far. If you want to I can add an HDL-based example and use it to extend and verify the guide. Google