doc/README.SUSE: Add how to update the config for module signing
    (jsc#PED-5021)
    
    Configuration files for SUSE kernels include settings to integrate with
    signing support provided by the Open Build Service. This creates
    problems if someone tries to use such a configuration file to build
    a "standalone" kernel as described in doc/README.SUSE:
    
    * Default configuration files available in the kernel-source repository
      unset CONFIG_MODULE_SIG_ALL to leave module signing to
      pesign-obs-integration. In case of a "standalone" build, this
      integration is not available and the modules don't get signed.
    
    * The kernel spec file overrides CONFIG_MODULE_SIG_KEY to
      ".kernel_signing_key.pem" which is a file populated by certificates
      provided by OBS but otherwise not available. The value ends up in
      /boot/config-$VERSION-$RELEASE-$FLAVOR and /proc/config.gz. If someone
      decides to use one of these files as their base configuration then the
      build fails with an error because the specified module signing key is
      missing.
    
    Add information on how to enable module signing and where to find the
    relevant upstream documentation.