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>