HACKING.md: explain a bit more about how to build xdgmime for testing
Note that the submodule must be built before configuring the shared-mime-info build, and that xdgmime should be built using its Makefile, not Meson.
Note that the submodule must be built before configuring the shared-mime-info build, and that xdgmime should be built using its Makefile, not Meson.