rust: add note about excluding files
    
    There are few reasons for this:
    * BRP scripts are run with full buildroot (not taking %exclude into
    account) which slows down build process and might fail because some brp
    not allowing such file to be present in there (hello unversioned Python
    shebangs).
    * Forwarding such exclude line to upstream will make possible to remove
    any downstream patching in future. This is beneficial for upstream
    because their archives on crates.io will be smaller which people like.
    
    Signed-off-by: Igor Gnatenko <ignatenkobrain@fedoraproject.org>