File README.usrmerge of Package kmod

The Tumbleweed kmod and kernel are patched to install modules into
/usr/lib/modules (the --with-module-directory configure argument.)

This is not compatible with upstream kernel, and upstream rejected patch to
make the kernel build system detect this. Suggested solution is to use

make MODLIB='$(INSTALL_MOD_PATH)/usr/lib/modules/$(KERNELRELEASE)'

when building the upstream kernel.