Document where to find symbols and debug info for sel4-kernel-loader

[nspin]

The output of sel4-kernel-loader-add-payload is an ELF file that only has program headers, and no section headers. The original sel4-kernel-loader.elf, which is used as input to sel4-kernel-loader-add-payload, has debugging info which can be used to debug the final program image.

As pointed out in #179 and seL4/seL4#1313, this is not intuitive, and thus should be made clear in documentation.