[Libguestfs] [libnbd PATCH] rust: Add cargo as an optional build dependency in README

[Richard W.M. Jones]

Thanks, I have pushed this as commit 41e921b554.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
nbdkit - Flexible, fast NBD server with plugins
https://gitlab.com/nbdkit/nbdkit