Sources: Add section about conditionalization
    
    A common mistake when writing multi-distro/multi-release spec files is
    to conditionalize the source tarball or patch entries. While this may
    generally work in Fedora because of how we always rebuild the SRPM for
    the target build, it's bad practice and prevents the reuse of SRPMs on
    other releases.
    
    This new section requires that all sources be shipped in the SRPM and
    then their use (rather than presence) is conditionalized.
    
    Signed-off-by: Stephen Gallagher <sgallagh@redhat.com>